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Credit: Bob Paz
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Interview of David Hitlin by David Zierler on May 6 & 25, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Interview with David G. Hitlin, Professor of Physics at California Institute of Technology. Hitlin discusses his thesis work on high-resolution muonic X-ray studies with his advisor and mentor Chien-Shiung Wu, and his subsequent transition to elementary particle physics at SLAC. He relates his experiences with kaon physics as a member of Mel Schwartz’s group at SLAC and Stanford. As a member of the Richter group at SLAC he worked on the Mark II experiment and then founded the Mark III experiment at SPEAR. After moving to Caltech in 1979, he worked on the SLD experiment at the SLC and then as founding Spokesman of the BABAR experiment at PEP-II. The interview ends with a discussion of his current involvements with the Fermilab experiment Mu2e and the nascent SLAC experiment LDMX.
Part 1 – May 6, 2021
Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It's May 6th, 2021. I'm delighted to be here with Professor David G. Hitlin. Dave, it's great to see you. Thank you for joining me today.
I'm happy to do this. It will be interesting.
Dave, to start, would you please tell me your current title and institutional affiliation?
I'm Professor of Physics at California Institute of Technology.
How long have you been at Caltech?
Dave, a question we're all dealing with right now, how has your science been affected one way or the other in this year plus in the pandemic, being away from your colleagues, but also perhaps not commuting, not doing things where you're moving around all the time? In what ways has that given perhaps more bandwidth to work on problems you perhaps might otherwise not have?
Well, it's certainly been different. We've adapted as best we can. There have been protocols put in place about how often we can go into the lab, etc. I haven't been on the campus in more than a year because the older people were just banned. The younger people were allowed to come into the lab one person per so many square feet at a time. So, we've been able to get some work done in the laboratory. Of course, we haven't been able to go to the national labs, to Fermilab or SLAC or the places I would normally go to, and that has an impact. Conferences have turned into Zoom meetings, which sort of works, except that it sometimes means that the interesting talk is at 4 o'clock in the morning, so it's not an ideal situation. All of that will come back to life slowly, and hopefully soon.
And just as a snapshot in time, circa May 2021, what are you working on right now? What's been most compelling to you in recent weeks or months?
Well, I'm involved in two experiments. Well, really, three, but the big experiment we're doing at Fermilab is Mu2e, a search for muon converting into an electron without any compensating neutrinos in the field of an aluminum nucleus. This is a process which is basically forbidden in the Standard Model, but many other theories say we could be on the verge of seeing it, and that would be extremely interesting. So, that takes a fair amount of time. The other thing I've been doing quite a lot lately is we're trying to get approval for LDMX, a new experiment at SLAC to search for dark photons—a type of dark matter that exists in certain theories. I'm the technical coordinator of the experiment, responsible for making sure that we have the right sort of equipment with the right capabilities, and that they all fit together, and that we can afford it, and all this stuff. So, we have been given a small amount of seed money by DOE, and we're in the process now of trying to get the thing approved as a real proposal. So, that's taking quite a lot of time at the moment.
It's been fun in these past few weeks getting everyone's snap judgment on all of this excitement surrounding the g-2 muon anomaly at Fermilab. What's your perspective on this, and what are the chances that we're seeing new physics right now?
Well, we certainly don't have an explanation; what we learned from the new experiment is that the old experiment is right, so in some sense, it isn't news, but it's a very important confirmation. Unless we come up with some difference in one of the inputs to the theoretical calculation, it very likely is new physics, which is great. I don't think anybody really knows what the next step is.
Just to go a little more broadly from that, what is your sense more generally in particle physics right now, with regard to the interplay between the guidance that theorists are providing, and where experimentation is based on that guidance?
Well, experimentalists don't just do random things. They do things because there's a theory to tell you to measure something, or that can be proved or disproved. There's always this interplay. It's always been that way since I've been in the field. What's different now is that it takes ten, or fifteen, or twenty years to do an experiment. You can start to design an experiment and get it approved in year one, and then in year twenty, you may have the answer and it may or may not still be pertinent.
And there's half a career right there in those twenty years.
That’s right. I'll tell you a story. I worked at SLAC as a postdoc in the group of Mel Schwartz, who won the Nobel Prize in 1988 for discovering that there were two neutrinos. He was also my undergraduate advisor at Columbia, and then I was a postdoc in his group at SLAC. He was a very interesting person; he was always thinking about the next thing. The experimental thing that was going on now no longer interested him, even though everybody else in the group was engaged in it; he was on to the next thing. Mel liked to bounce ideas off me, and we actually had, in the corner of my blackboard, a list of things that we would be working on if we were only a little smarter. For example, we really didn't know how to find gravitational waves in 1970, but we knew it was important, so we had it on the list, along with other important questions. This was a conversation we had all the time. It was extremely interesting, and I learned a lot from it.
One day Mel came into my office, and he said, "What if you thought of an experiment that was for something really important, but it would take a hundred years to get the answer? Would you do the experiment?" That was amusing, so we discussed it for a while. My point is, we're actually now there. It doesn't have to be a hundred years; it just has to be a career's worth, or if you're 40, it has to be 25- or 30-years’ worth. And that's what major experiments take now. So, we are in a situation where you can say, "I'm going to start this experiment, and I may or may not retire before there's an answer." But I think the ethos of the field is, well, if it's interesting, you do it. You start it, and you hope you live long enough to get the answer, or somebody else will get the answer. But that was quite a prescient concept that Mel had back in those days.
Dave, let's take it all the way back to the beginning. Let's start first with your parents. Tell me a little bit about them and where they're from.
My father, whose name was Maxwell, was born in Brooklyn, and lived in New York his entire life. He had a master's degree in business administration from NYU, but got it during the Depression, in 1932, and never actually worked in that field. His hobby was amateur radio (W2GEN: he was proud of the early call sign), that he pursued his whole life. He managed to turn that into a career. He became an electronic engineer and worked for various electronic companies in the New York area for many years, and always had a little workshop in the basement with things that he would be building. He built our first three television sets. We had a five-inch television, which was interesting because it had electrostatic deflection; The screen was based on an oscilloscope, so it had electrostatic scanning of the raster, as opposed to magnetic scanning, which was the way most TV cathode-ray tubes work. He built that, and then he built a 7-inch version, and then he built an 11-inch one.
So, we were the first people to have TVs on the block. Kids used to come in to watch all the time. He never got around to putting any of them in cabinets, but they worked fine. My mother was from Louisville, Kentucky. Her name was Martha Lipetz. She had family in New York, so they somehow got together—I don't know exactly how. They were married in 1939, and then lived in New York and Brooklyn. So, that's where I grew up.
Where in Brooklyn?
A couple of places, but the place we lived the longest was in Crown Heights, which was quite close to Ebbets Field, where the Dodgers played when they were in Brooklyn. Perhaps ten blocks from Ebbets Field. I was a Yankee fan, so that was difficult, but I survived.
Was Crown Heights mostly a Jewish neighborhood at that point?
Largely, yes, but it changed over time as these things do. So, my parents moved to Queens in the early ‘70s.
Which was probably considered a suburban move to some degree.
Well, not quite. They moved from a house into a condo. They were older, and they felt it was the right thing to do.
What schools did you go to growing up?
If you're asking me, was it P.S. 57, or something, I don't remember. But I did have a somewhat unusual elementary school career in the following sense. In the 5th grade, I was sent to a school for which I had to take two buses, to a class that was called IGC, Intellectually Gifted Children. I don't think they would dare give a class that title anymore. But it was a very interesting class. It had about 24-25 students, not sitting in rows of desks, but with tables with two students at a table, which is not unusual now but was quite unusual then. The 5th and 6th grade were in the same class. The way it worked was that your time was basically free, except there would be lessons in which the teacher (Mrs. Wallendorf) would present a standard classroom lesson part of the time. Certain things had to go in a certain order; there was a math class for the 5th graders and then one for the 6th, but other things like history, were done together; you may have gotten units in reverse order which didn't really matter. The main point about it is that you were independent, and you had to “fulfill a contract.” Every week you had to turn in a lesson on a given subject, and it was your responsibility to just do it on your own time, whenever, and hand it in. There was a library in the back of the room, and if you wanted to read, you could go back there and read. There was a nature room across the hall with frogs and salamanders and things. I was in charge of the nature room. That was for the 5th and 6th grade.
I then went to Halsey Junior High School, which also required a subway ride, and that had another unusual arrangement in which you did 7th, 8th, and 9th grades in two years. It was not mixed up the way 5th and 6th IGC class was, but you nonetheless did three years' work in two. I then went to Brooklyn Technical High School, which is one of the “elite” schools in New York where you take a test for admission, like Bronx Science, Stuyvesant, Brooklyn Tech, Music and Art, etc. That was also slightly unusual because I had done the 9th grade in junior high school—9th grade is normally part of high school, but Tech’s rules were that if you'd done the 9th grade in junior high school, then you had to take additional summer classes to make up for things that were part of their curriculum but were not part of the junior high school curriculum, or you could just stay an extra half year. Since I was younger than everybody, having done the three years in two, I just stayed an extra half year.
And that would be January of what year?
'59. Because I didn't go to summer school, I had this extra term to complete. So, I graduated in January, and applied to colleges. My mother insisted that I had to go to college in New York. So, I agreed.
Would that include even Harvard? Sometimes I've heard Harvard would be the exception.
No, that never came up. I didn't apply to Harvard because of that. I applied to Columbia, and I applied to Brooklyn Polytechnic Institute. I don't know if that still exists. I think it's subsumed into some other—
I believe NYU took it over, if I'm not mistaken.
Could be. I could get in there in February, but to matriculate at Columbia, I would have to wait until September. Brooklyn Polytech offered me full scholarship; they were handing me the whole store. But I decided I would wait and go to Columbia in September.
Dave, was it specifically physics that you were focused on as you were thinking about undergraduate, or you were still open at that point?
No, I think I was pretty determined to do physics at that point. I mean, I hadn't—
I wonder, also, in 1959, did you sense the impact of Sputnik, that this was creating new opportunities in science generally, and physics particularly?
Absolutely. Sputnik made an enormous impact. It's hard to overestimate the impact it made on everything. In fact, I took Russian as an undergraduate because that was going to be the coming thing, of course. Before we go on, I'll tell you a story about Brooklyn Polytech. I spent ten years at SLAC, and there were three Nobel Prize winners on the faculty. One of them was Martin Perl, who discovered the tau lepton. One day, we were sitting around a lunch table, and I mentioned what I just told you about having had a choice of going to Brooklyn Polytech or going to Columbia, and I finished by saying that decision made an enormous difference in my life, which is true. And Martin just looked at me and said, "I went to Brooklyn Polytech." So, he put me in my place.
Obviously, you don't have anything to compare it to, but is your sense that your education in math and the sciences at Brooklyn Tech was a step above what you would have gotten at a P.S.?
Yes. Without a doubt. It was a good school. It was an interesting school in the sense that it had origins which I think went back to the '30s that had some relation to the German high school system where they track people into academic and technical areas, where you become a mechanic or technician, instead of going on to a four-year college. It had an aero course, an electrical engineering course, a mechanical course, and also a college preparatory course. Most people took the college preparatory course, but some took these other things; I don't think at that point that this meant they didn't go to college, but it was more of a concentration on technical things, and less on English, humanities and whatever, I suppose.
What were your initial impressions of the department of physics when you got to college?
Well, Columbia was the center of the world in physics at that point. It hasn't been for a while, but it certainly was then. It was great; I had a remarkable time there.
Who were some of the luminaries that made you recognize immediately what a special place Columbia physics was?
Well, I remember the first lecture I ever had in freshman physics was taught by Leon Lederman. He wasn't the teacher in the class. That was Bill Havens, who did the rest of the term, but he was out of town that day, so Lederman did the first lecture, which I've never forgotten because he was great. He was funny, he was clear, he held your attention. There was just a feeling that things were going on, important things were happening. I always found that people were approachable. The rest of the students were really terrific. That was actually an adjustment, because I was used to being the best. When I graduated from Brooklyn Tech, I got a whole bunch of medals and awards. When I got to Columbia, it was clear that there were a lot of people who were smarter than I was. Which was fine as far I was concerned. It didn't bother me at all, but it was a different world, the big leagues.
To the extent that, from your vantage point as an undergraduate, hearing what the professors were talking about, what were some of the most exciting things that were happening in physics in the early 1960s?
One of them was in fact the preparation of the two-neutrino experiment, which ran at Brookhaven in 1962, if I remember correctly. It was being built when I was an undergrad, so we would hear about it. It was a very big experiment at the time. It was the first accelerator neutrino experiment; that was one of its great innovations. Previously, neutrino experiments, of which there had not been many, had used neutrinos from reactors. So, that was interesting and exciting. Chien-Shiung Wu, who eventually became my thesis advisor, was doing an experiment to prove a theorem about the conserved vector current at the time. That turned out to be a very important result. There was just something happening all the time. You had to be interested and plugged in to understand that and to learn about it, but you could. One of the things that was quite important was coffee hour. I did this more when I was a grad student, but as an undergraduate sometimes, there was a coffee hour on the 8th floor of Pupin at 3:30 every day. It was just a big room with some tables and a few chairs and blackboards, and coffee and cookies. And everybody went every day. It was remarkable. All of these people, either who had Nobel Prizes, or were going to receive Nobel Prizes, were standing around talking about physics. It was an education in itself just to be in the room.
What were some of the laboratory experiences as an undergraduate that may have been formative for you?
I didn't do any research as an undergrad. I was working during the summer as a draftsman/illustrator to earn money for tuition. I had a scholarship, but I had to earn the rest of the money. I was able to get a job like that because drafting, at many levels, was part of the curriculum at Brooklyn Tech. We had mechanical drawing, different levels of sophistication, each year. So, I had a skill, and was able to get summer jobs as a draftsman and technical illustrator. For example, when I graduated in January '59 and was not going to be able to get into Columbia until September, I worked as a draftsman for Western Electric, which was the manufacturing arm of AT&T at the time. Their building was on West Street downtown, right near what became the World Trade Center. It was a rather interesting and intricate job because they were in the process of changing over from the old switchgear in their exchanges to new transistor-driven crossbar switches. The revisions had to be done on the fly, making changes without interrupting telephone service.
We had old drawings from the '20s, which were on vellum. I don't know if you know what vellum is. Modern vellum is paper, but this vellum was sheepskin, which was sized with starch, and then drawn on with India ink. We modified these old drawings to insert the new switchgear, using the old India ink technique. The way you would erase something would be to take a razor blade and scrape the ink off the vellum, and then you would draw the new items with India ink. That was an education; I did that for six months and earned some money too. I didn't do any research as an undergrad. The first time I was in a lab was between my senior year and first year of grad school. I can go back to that, but I decided to go to graduate school at Columbia, so in the summer of 1963 I worked in the lab of Sven Hartmann, who did nuclear magnetic resonance.
Dave, is this to say that as an undergraduate you were still open between theory and experimentation?
No. I clearly knew I wanted to be an experimentalist. It's just that I basically didn't have the opportunity in the first couple of years, because I lived at home, and I had to take a long subway ride back and forth every day. As a junior I moved into a small room in an SRO because I needed the time to study, and I didn't have time to do any research in a lab. I worked for Sven Hartmann between senior year and first year of graduate school, which was interesting. The other student working in the lab with me at that time was Norman Christ, who is now a theorist at Columbia. He became a student of T.D. Lee and became a well-known theorist. But he worked in Hartmann’s lab with me, and it turned out he was a complete whiz in electronics. Something I wouldn't have guessed, but he was very good at it and very helpful.
Did your mom's rule of staying in New York apply to graduate school also?
No. I applied to various places—I got into Yale, Wisconsin, and Columbia. I couldn't decide between Yale and Columbia, so I flipped a coin, and it came up Yale, but I decided I wanted to stay at Columbia. And I'm glad I did. The situation was actually kind of interesting. The department always wanted to have a few undergrads stay on for graduate school. When they did the statistics, they found the number who stayed was shrinking. So, in that year they made quite a lot of attractive offers to the Columbia undergrads. You always have to admit more people than come in the end. I know that very well because I've been chairman of the graduate admissions committee at Caltech several times. So, you have to admit twice or three times as many people as are actually going to enroll; it's a bit of a crapshoot. They made a lot of attractive offers to the Columbia undergraduates, and basically all of them decided to stay at Columbia. It was something like ten, maybe a third of the class, which really surprised them.
Dave, as an undergraduate, did you have a good sense of who you wanted to work with, who would be your graduate advisor had you gone with the decision to stay at Columbia?
Not really. There was an interaction that in retrospect may have something to do with that. I took a nuclear physics course as a senior from Wu, and one of the requirements in the class was to write a term paper. I wasn't sure what to write about, so I discussed it with her, and she suggested that I write a paper about lithium-drifted germanium photon detectors. I was interested in electronics, and this was a hot new thing, so I wrote a paper delving into Ge detectors in some detail. It was actually a pretty good paper. And that was the end of that; it was just something I did in the course. Eventually, I wound up working for Wu, and building germanium lithium-drifted detectors for my thesis experiment. Perhaps she had cleverly set a trap.
Now, let the record state, we're only capturing the audio, but the video demonstrates that Dave has the Madam Wu stamp as his Zoom background right now.
Yes, that stamp was just issued in February, so it's a good Zoom background for a while.
Dave, what was Madam Wu like as a person?
Interesting. She had a rather fierce reputation, and over the years, I came to know some of her older students. She was rather demanding of them. They always kept in contact with her, but several of them felt that they had had a harrowing experience. I did not. I had just a wonderful experience with her. She was demanding; that was fine as far as I was concerned. She was a real model of how you do experimental physics. She was in her 50s when I was her student, so she was not in the lab day-to-day tinkering with oscilloscopes and that sort of business, but she was very incisive about what you should do, what you should check, what could be wrong, and very interested in the calculations that had to go into these things.
Her standards were high. If you performed, which at least in my era, everybody in the group did, there were no controversies, and she was great. She was warm, she was friendly, we would go to Chinese restaurants with her, which was an interesting experience because she was a superstar in the Chinese community. It was like Elvis had walked into the restaurant. And there were parties at her apartment once in a while. When we were running experiments at the Nevis cyclotron, we would run in typically two- or three-week intervals, quite intensively, 24 hours a day, 7 days a week, and she would occasionally come to Nevis and bring us lunch.
How hands-on was she as a mentor? Would you work closely with her, or would it be weekly meetings? How did that play out?
I don't recall anything as formal as weekly meetings, but we had lots of interactions. She wanted to know what was going on. Either there was something that you needed or wanted her advice about, or she would ask you. So probably with weekly frequency or maybe even more than that. My office, on the thirteenth floor of Pupin, was actually an anteroom to her office; you had to go through my office to get to hers, so I met a lot of interesting people.
How closely was your research related to what she was working on at that point?
This is what she was working on. She had many demands on her time, but she was doing two primary experiments at the time. One was my thesis experiment on muonic X-rays, which we can talk about in more detail. The other was a search for double beta decay in calcium-48, which was a hot topic at the time, and is still a hot topic because it's never been found. I can tell you a little aside about that. The Ca-48 experiment consists of looking for two electrons emitted in a very rare process. This was not my thesis, but other people in the lab right in the next lab from ours were doing this, so we had lots of back and forth about what was going on. The idea was to see whether there was a decay of calcium-48 that produced either a continuous or a monoenergetic electron spectrum. That would be the case depending on whether neutrinos were ordinary or Majorana, whether neutrinos were their own antiparticles.
Can you explain that a little more, Majorana as the other option?
A Majorana neutrino is a neutrino that is its own antiparticle, which means if you write a Feynman diagram, you can wind up with a different final state than if the neutrinos are distinct. So, the signature is, again, whether you have monoenergetic electrons or a continuous electron spectrum in the final state. They (Jack Ullmann, Keith Bardin and Peter Gollon) were doing this experiment. They actually didn't finish it until a few years later. Eventually they took the experiment to a salt mine under Lake Erie in Cleveland to reduce cosmic ray backgrounds. There are about ten different isotopes that can undergo double beta decay. Wu had chosen calcium-48 to do the experiment. But there is another possible isotope, germanium-76, which is a major component of stable germanium. One day I was paging through Phys Rev Letters, and there was a paper by Fiorini, from Milan, about a new limit on double beta decay in germanium-76. I wanted to really kick myself because I was the maker of the best germanium detectors in the world, the largest most sensitive detectors. In the next room were the people doing double beta decay with calcium-48. I never made the connection that I could have done, with very little effort, an experiment more sensitive than what Fiorini did.
Well, just to be fair, your neighbors didn't make that connection either. It wasn't just you.
Yes, but I didn't blame them; I blamed myself. I remember to this day how I kicked myself at the time for not making that simple connection.
We should talk about muonic X-rays at this point.
Muonic X-rays were actually discovered at Columbia, at the Nevis cyclotron, by Val Fitch and James Rainwater in 1952. They were predicted in 1947 by John Wheeler. Both Fitch and Rainwater went on to win Nobel Prizes for other things; Rainwater for a theory paper, which is actually worth noting, and Fitch for the discovery of CP violation in kaon decay. Neither Nobel award had anything to do with muonic X-rays. The case of Rainwater was extremely interesting because the Nobel Prize in 1975 went to Bohr—that's Aage Bohr, Niels Bohr's son—Ben Mottelson, and Jim Rainwater. I would guess that Rainwater was the most surprised person on the planet when he got the Nobel Prize. But the reason is clear if you look in the literature. Bohr and Mottelson made a career out of working on the collective model of deformed nuclei, which was basically an adaptation of Maria Goeppert Mayer's shell model to a deformed potential. A lot of nuclei have permanent quadruple deformations, which determines many of their properties. They wrote two books about this and worked on it for their entire career. That's what the Nobel Prize was for.
However, in the early '50s, Aage Bohr was a postdoc at Columbia, and he shared an office with Rainwater. Rainwater wrote a short paper, about a page long. This was before there was Phys Rev Letters; there was Physical Review Comments, which were short letters and comments in each issue of Phys. Rev. Rainwater’s paper said if you took the shell model and put it in a deformed potential, then you could learn a lot of interesting things. He showed a simple example calculation, and that was the end of it. He never did anything else in this area, because he was actually an experimentalist. But he had shared an office with Bohr, so Bohr knew about this. Bohr then took this idea and made a career out of it. The Nobel Prize committee did its homework and decided to give Rainwater a piece of the Nobel Prize, which was, I think, fair. He had the idea, and ideas are what they really give Nobel Prizes for.
For your own research, how did you know you had enough? When were you ready to defend?
My thesis? That was actually unusual. Let's go on and then I'll get there. The muonic X-rays found by Fitch and Rainwater are gamma rays in the range of, typically, 1 to 6 or 8 MeV. The detector of choice for doing this kind of experiment was thallium-doped sodium iodide, a scintillating crystal; you view the scintillation light with a photomultiplier tube. It was the best way to do these experiments, but its resolution was not very good. If you were looking at something that's a narrow line, because of the poor resolution you saw a gaussian distributed over a large range of energy due to the poor resolution.
What was interesting about these germanium lithium-drifted detectors was that even though they were much smaller, they had vastly better resolution, by a factor of 100. The idea then was, and several people decided to do this at roughly the same time, that with germanium detectors you could see the muonic X-ray fine structure, or even the hyperfine structure due to quadruple deformations, and that's what I did my thesis on. Everyone with a cyclotron did this: in Switzerland, in Chicago, in Virginia, but it was first done at Columbia by Sam Devons, who had recently come to Columbia from Manchester. In fact, I'm in the middle of writing an article for the Royal Society. When RS Fellows die, they have an article written about their career. I was asked to write such an article about Sam, because he worked on my thesis experiment. I knew him quite well. Together with some people from Brookhaven he used a small lithium-drifted detector and did an experiment at Nevis on two isotopes of molybdenum and was able to resolve the fine structure and measure the isotope shift. Wu then got interested in doing this and they formed a collaboration. Devons' group, which was just one postdoc and a student, Wu's group, which was larger, and Rainwater. Wu thought it would be politically useful to have Rainwater, the discoverer of muonic X-rays, on the experiment. He didn't do much, but he was interesting to talk to. He lived right near Nevis. He had been a major contributor to the building of the cyclotron and knew everything about its operation. In the middle of the night, 2 o'clock in the morning sometimes, you would see the beam get better. The reason was that Rainwater couldn't sleep, so he would come in, shove the cyclotron operator out of the way and tune up the beam; he could do it better than they could. That was actually his main contribution to the experiment.
In any case, we had this powerful contingent of Devons, Wu, and Rainwater on the experiment. We embarked on a quite ambitious survey of many isotopes: basically, everything Wu could get her hands on. The way that worked was quite interesting, too. These separated isotopes are produced at Oak Ridge, where there's a repository of separated isotopes. They make them or made them—I don't know if they still do—using what were called calutrons. These were small mass spectrometers, basically miniature cyclotrons, that were originally used to separate uranium isotopes, along with gaseous diffusion, which proved to be a better method in the end. But they kept these calutrons in operation and then used them to separate other isotopes. They built up a store of isotopes which ranged from fractions of a gram to hundreds of grams of various isotopes. You could get neodymium-142, 143, 144, 145, 146—I think there is no 147, 148 and 150, just as an example. And then, tungsten-182, 184, 186; all kinds of separated isotopes. The kicker was that they're horrendously expensive. Most people who needed to use them used milligram amounts, but we needed tens of grams, or hundreds of grams, of each of the isotopes. You couldn't afford to buy these quantities; it would have been literally many millions of dollars. Wu made a deal with Gregory Ragosa, the head of the nuclear physics program office in the DOE, which was the AEC at the time; they would lend us all of the material we needed. They came in standard chemical reagent bottles. We would make a funnel out of filter paper and pour the samples into lucite boxes that we made in the appropriate size and use them as targets for the muons. We would then put them back into the reagent bottles. They would weigh the bottles before and after and they would charge us only for any loss; a small amount might stick to the filter paper, for example. That was the deal, so it cost us only a small amount of money to use this unimaginable wealth of separated isotopes.
There was an interesting incident along the way. We would take data at the cyclotron for several weeks every several months, making improvements and obtaining new isotopes in between. In the interim we would take the germanium detectors and isotopes back to Columbia. The Nevis cyclotron was in Ardsley, up on the Hudson River, about 25 miles north of the campus. We would drive the detectors and the isotopes back and forth. We kept the isotopes in a small cardboard box. At the start of one run, I had them in the back of my car, and was driving to Nevis when I was stopped by the state police; my car registration had expired. The reason for that was that New York was making a switch between all licenses renewing on January 1, to spacing renewals over a year. Naturally, because of the bureaucracy, or the mail—this was mid-January, or so—my new tag had not arrived. I had a choice of not driving or taking my chances, and I drove. The trooper gave me a ticket, but then wouldn’t let me go on; he impounded the car. They towed it to a garage—I think he had a deal with the garage; a kickback for each car he impounded. I called Nevis and had them send a car to pick me up. I insisted on riding in my car while they towed it, as they wouldn't let me drive; they had to tow it, of course. Nevis sent a car, and we brought the germanium detector and the isotopes to Nevis. It took me about a week to get my car back. That was a close call, but it's one of the vagaries of doing an experiment. We did these experiments for several years, improving the detectors as we went, making them larger, more efficient and with improved resolution.
Dave, is this a Columbia-only endeavor? Are you in contact with people at other institutions?
In that experiment, we all were from Columbia. There were often collaborations between institutions, but that experiment was just Columbia. You were asking about my thesis. I did a very unusual thing. We had data, and I had analyzed the data, and I was going to stay a little bit to write things up for publication, so they gave me a job as an instructor, an academic rank below assistant professor; you teach, but there's no tenure at the end of the rainbow. By that time, commercial detectors had outpaced our homemade ones. They were much larger, more efficient, with higher resolution. I decided to go back to Nevis and do all my thesis runs over again with the new detectors. I didn't write my thesis in 1967 as I was supposed to, but I was still an instructor, and I was teaching an introductory physics course in the School of General Studies, Columbia’s night school, which was actually a lot of fun. The students were a quite different group than Columbia College undergraduates. Older, more mature, in some ways more serious. So, I had a good time doing that; the students kept calling me "Doctor," but I wasn't a doctor, because I hadn't filed my thesis. We redid all the experiments, and then I wrote my thesis in 1968. I had done a good job in General Studies, so they asked me to teach the same introductory physics course in the College. That was not nearly as much fun, because the students were, of course, much brighter, but a lot of them were premeds, and they argued for two more points on a homework problem, and that kind of stuff, which I don't have a lot of patience for. So, it wasn't nearly as good an experience as teaching in the School of General Studies.
We had very nice results and showed the results at several conferences. During these years I had the opportunity to go conferences in Williamsburg, in Ottawa, in Gatlinburg. Wu was generous about having the young people make presentations at conferences, which is something I've never forgotten, and try to do as well. Typically, those are the people that do the hard work, and the ones that need the recognition. We published a whole series of short papers as we went along. Then, we published two comprehensive Phys. Rev. papers, one based on my thesis on the analysis of hyperfine structure in a whole series of deformed nuclei, and a companion paper on work done by my fellow student Eduardo Macagno, on isotope shifts. We took all the data together and analyzed it together; he then did the isotope shifts and I did the deformed nuclei. Eduardo then had an interesting career. He became a biologist and stayed at Columbia. Eventually he became Dean of the Graduate School of Arts and Sciences, and then about a dozen years ago, moved to San Diego where he had an equivalent position. We worked very closely together on our theses and have kept in touch a little bit since then. We did the definitive work in this area; there was some work in the succeeding few years, and then all of the cyclotrons were turned off, so that kind of physics isn’t done much anymore. One unusual thing is that my current experiment, Mu2e at Fermilab, is a search for m to e conversion in the field of aluminum nucleus, basically an aluminum muonic atom. I find that I know things about the details of muonic atoms that the rest of the particle physicists don't actually know much about. So, that's been an unusual turn of the wheel back to the beginning. Okay, so where were we?
The question of when you knew you were ready to defend.
Well, as I said, we had everything done and analyzed, and I could have defended a year earlier than I did, but I wanted the better data. There were a lot of calculations that had to be done. Wu always made sure to have access to the appropriate theorists. When she was doing beta decay, her famous series of experiments on parity, she saw to it that there was a theorist named Morita, from Japan, who was in residence at Columbia, because she knew the importance of the various theoretical calculations that had to be done. When we came to muonic X-rays, she repeated that idea. We had a young theorist, Roger Barrett, who came from the University of Sussex, who did the definitive work on solving the Dirac equation to derive nuclear parameters from muonic X-ray spectra. I used his program in my analysis—I made modifications to it, but basically it was his program, to extract all the nuclear parameters. You start with the measured energies of all the fine and hyperfine structure spectra, then solve the Dirac equation, modifying the nuclear size and shape parameters until you get a fit to the data. Columbia's main central computer at the time, an IBM-7094, a 32-bit machine, couldn’t do these calculations with sufficient precision to get consistent results. There was a CDC-6600 computer, a 64 bit machine, at the Courant Institute at NYU, downtown. Wu arranged to get us some time on that computer. The program was on two boxes of IBM punch cards. You've probably never seen an IBM card.
I have not. I've heard about them; I've never seen one.
They're about that big (indicates size on Zoom) [They are 7 3/8 x 3 ¼ inches]. They have 80 columns of small rectangular holes and were the medium you wrote the programs on. Fortunately, they let you store the boxes of cards at NYU; I didn't have to take them back and forth on the subway. I lived on 75th Street—7½ West 75th Street, which I've always found amusing. I would take the subway to NYU first thing in the morning, submit a bunch of jobs, and come back up to Columbia. In the evening, I would go back to NYU, look at the results, submit some more jobs, and do this each day for a couple of months, until we had all the results. I then wrote this up for my thesis. That was before word processing. It was all done on a typewriter, so when you wanted to change something, it meant retyping a page.
Or real cut and paste.
Real cut and paste. I can tell you another little story, which I probably shouldn't.
When we would write our papers, Wu was very involved in the writing and would sometimes write a first draft, or later rewrite a paragraph. Her written English was not fully colloquial, so I would go over the text and make changes. She didn't like that. So, my solution to that problem was eventually to just edit but not show it to her. That worked; our papers wound up being literate.
Dave, besides Madam Wu, who else was on your thesis committee?
Rainwater, Devons, T.D. Lee—I don't recall the others. Columbia had a complex system of exams after you handed in your thesis. After the thesis was submitted, there was a department oral exam, and then there was a separate “Low Library” oral exam. The departmental oral was given by people from the department, and then the Low Library oral had most of those people and then several others from different departments. That’s not unusual. When I was at Stanford, we had the same system for the equivalent of the Low Library exam, which was kind of fun. They asked you to sign up for what departments you’d like to be the outside members on. I chose things like music and biology and things of that sort. One time I was on a biology exam, and the basis of the experiment they were doing was chromatography, in which you measure the rate chemicals migrate in an electric field to separate the various components of DNA, etc., They don't do it quite that way anymore, but that's what they used to do. I asked the student, “How does the chromatography work?” “If I double the voltage, how much faster does the chromatography happen?” It’s very simple; it’s Stokes’s law. He had no idea, nor did anybody else on the committee, which included two Nobel Prize winners in biology.
Did you have much direct contact with T.D. Lee?
I had a graduate particle physics course with him. He later wrote a book based on his lectures. The course was marvelous; he was as clear as day. You walked out of the room thinking you knew everything, until you went back and looked at your notes and saw that there was more to it than that. He was a marvelous teacher; the kind of teacher who would write on the board, and turn around and keep talking, while watching to see when people stopped writing, and then he would write some more. He was concerned that you were gathering what he was saying. He was great about that.
He was, one-on-one, what’s the word? Imperious is probably the word. That didn't really apply to me in any way, but you could see that in his interactions with others. I mean, he was T.D. Lee, a formidable character. I'll tell you something more about the exam situation at Columbia. As I said, there was this oral exam in the department, and then the Low Library oral exam. There was an additional feature of the exam process, which was kind of in flux when I was there. For many years, when you handed in your thesis, you next had to take a written examination on physics, on partial differential equations, on electrodynamics, etc., a really difficult written exam. Then, if you passed, you defended your thesis. Many of the faculty decided that this was out of step with the times, that it was no longer a good idea. They changed the requirement to a requirement to write a series of review papers. I'll tell you something Mel Schwartz told me about this, because he was at the faculty meeting where the decision to make this change was made. I. I. Rabi, the person who created the great Columbia physics department, had already retired. He was emeritus, but he came to faculty meetings. He was very upset at the idea of not having this rigorous written final exam, and apparently, at this meeting, according to Mel, said, "If you do this, I will have witnessed the end of the physics department I created, and I will never come to another faculty meeting."
Not to mince words, or anything.
He never minced words. I knew him a little bit.
Why this reaction?
I don't know. It was the right decision, but it certainly upset him. Mel ended the story by saying "And he was right." I don't think that was true, but it was the way he ended his recitation of the incident. I was in the first cohort that had to write these review articles instead of taking a final exam. The various members of the committee proposed review article topics. I had three articles to write, which was a non-trivial job. It was enlightening, because you go into the literature and read a whole bunch of papers and then write a review article. What I learned from that is that over those last five years or so, I had become a physicist. You could do that. You were thinking hard about just your little thesis problem, but in doing so over the years you had become a physicist.
You're saying it's a slow transformation, and that you don't see it while it's happening, but in a moment like this it dawns on you.
Right. You could just read the literature, understand it, synthesize it. It was a revelation. I can even tell you at this remove what the articles were. Two of them were posed by T. D. Lee. One was to review the status of various conservation laws: parity, charge conjugation, and time reversal in the strong/weak electromagnetic interactions. So, you had to read the literature, find the best experiments, put their limits into some compatible structure, and then make a table. Lee was actually writing a paper about violation of charge conjugation symmetry around that time. The second one was to write about electron scattering. What he meant, I later learned, was to write about the deep inelastic electron scattering experiments that were happening at that point at SLAC, which later earned the Nobel Prize for Taylor, Kendall, and Friedman. But I took it to mean to write about electron scattering determinations of nuclear size properties, which were the equivalent of what I had done using muonic X-rays, which was certainly a possible interpretation. So, I wrote on the wrong thing, but did a perfectly nice job on a relevant topic. It didn't matter; it was just a cross-communication. The third assignment was to write about the cranking model, which was close to what Bohr, Mottelson, and Rainwater would get the Nobel Prize for. So, there were three substantial topics that one had to delve into to write these articles. They then questioned you on all kinds of physics, but with some emphasis on these review articles. I think that was actually an improvement; it was very much better than a written exam on partial differential equations.
Well, looking ahead, what postdocs were you considering at this point?
Well, I decided that I wanted to change from nuclear physics to particle physics. The reason, basically, was that in 1968, Wu sent me to a summer study at Los Alamos where they were considering building a new linear proton accelerator called LAMPF. When you want to start a new project, you gather the community and people look at all aspects of a possible program. I went to the summer study, and I worked on it for three weeks. It was interesting and it was fun to spend weeks in the New Mexico high desert. I'd never seen anything like that before. But I came back convinced that there wasn’t a career's worth of interesting science left to do in nuclear physics. And I think I was right. I mean, certainly the field of classical nuclear physics just died, and has now turned into this relativistic heavy ion stuff. I really didn't want to do that. I wanted to go into particle physics.
Did you see this as, in any way, a handicap? Was there new stuff that you would need to learn, that there was catch-up for you to do that other people who were graduate students in particle physics would not have had to do?
Sure. I wasn't worried about that, although it was clearly a concern for potential employers. I mean, the one thing physicists learn is how to learn. I've always said that. But the problem is, how do you get a job? So, I applied to lots of places, labs and universities, and I wasn't getting any positive responses. I got one offer for a nuclear physics job in Israel at the Weizmann Institute, with Gabi Goldring, who had been a collaborator of Sam Devons' at Imperial College. I guess Sam told him I was worth looking at, and he offered me a job. I only met Goldring once, twenty years later, at the home of a mutual friend at the Weizmann.
That was the only job offer I had, and then one day Wu said, "How's your job search going?" And I told her what I just told you. She had a mannerism: when she was thinking about something, or she didn't like something, she would shake her head from side-to-side. She did that, and that was the end of the conversation. A couple of days later, I was on the experiment floor at the cyclotron, which was a very noisy place, because it had motor-generator sets to provide clean power to the cyclotron magnet. The telephone ringer was a big fire bell. So, the fire bell rang, and it was Joe Ballam the Director of Research at SLAC, who said, "Why don't you come out and give a seminar?" A couple weeks later I went to SLAC for a seminar and interview. What had happened, I surmise—I don't know, but I surmise—was that Wu was a good friend of Pief Panofsky (W.K.H Panofsky, but known to all as Pief), the director of SLAC. They had known each other at Berkeley in the '30s, and they had remained friends. I'm guessing that she called Pief, and Pief then went to Joe Ballam, who called me.
So, I went out to SLAC in February 1969. There was a tradition at SLAC, I don't know if they have it anymore, of the brown bag lunchtime seminar in the Orange Room. At SLAC all the seminar rooms were named after colors. The chairs in the Orange Room were orange. The chairs in the Blue Room were blue. The chairs in the Yellow Room were yellow, etc. So, people would munch away at their sandwiches in the Orange Room while you gave your seminar. I talked about muonic X-rays, which was not something that the people at SLAC knew very much about, or probably even cared that much about, but that's what I had to talk about. And then you went around for interviews. They had a rigorous system of interviewing with each of the group leaders—there were groups A, B, C, D, E, F, G, and you would spend about an hour with each of them. Four of those people, if you count Mel Schwartz (who was at Stanford), wound up winning Nobel Prizes. Amazing. Each one was quite different, but extraordinarily interesting. Some of them probed you; some of them just told you what they were doing.
The interview with Dick Taylor, who did the famous deep inelastic electron scattering experiment, was particularly interesting because one of Wu's students, Luke Mo, who had worked on the conserved vector current experiment, was a postdoc in Taylor's group. One of the things needed to interpret the deep inelastic electron experiment is the radiative corrections for the fact that an electron can radiate a photon, in one part of the reaction or another; you have to make a correction for this. It's a rather complicated correction. So, Luke Mo got together with a theorist at SLAC named Paul Tsai, who died a couple months ago, and they wrote a paper on the radiative corrections to deep inelastic scattering. In so doing, however, they used some of the data before the experiment itself published the data. Dick Taylor was just fit to be tied, and here I was, a Wu student, and he was really pissed off at this other Wu student. So, he spent a good part of the hour complaining about Luke Mo.
Nonetheless, in the end, I got two job offers from SLAC. Normally, they would say, "Group C wants you." What happened was, Group A, Dick Taylor, despite his experience with Luke Mo, wanted to offer me a job, and Mel Schwartz also wanted to offer me a job. Mel had been my undergraduate advisor, so he knew me a little bit. Anyway, they both offered me a postdoc. What we decided was that I would come to SLAC, but I would not formally join either group. I would poke around, and then after a couple of weeks or a couple of months, I would decide. So, I found a desk in Mel's group and never left. I pitched into the K0 Spectrometer construction, started doing things, learning things, and just stayed there. The other choice would have been fine, too, but that's where I wound up.
What was your initial work at SLAC?
When Mel left Columbia, which was around '66, I think, he went to Stanford, not SLAC, but he had an arrangement by which his research funding would largely come from SLAC. He became the leader of SLAC Group G, but he was a Stanford professor. There was another person with a similar arrangement, David Ritson, also a Stanford professor, who led Group F. These two were sort of second-class citizens at SLAC. They didn't go to SLAC faculty meetings; they went to Stanford faculty meetings. They were adequately funded, but not at the level of the regular SLAC groups. When Mel arrived, he formed a group, with Stan Wojcicki, and did a couple of straightforward experiments. One was to measure the charge asymmetry in the semileptonic KL meson decay, and the other was to measure the two-photon decay of the KL. These experiments were basically done, except for a bit of analysis, when I arrived. They were building the K0 Spectrometer, which had a very large 100D40 dipole magnet to analyze momentum of outgoing particles in kaon decay. After we finished with it, it had a peripatetic life. I think it went to Brookhaven, then it went to Fermilab, where it was used in several experiments. It was a very expensive magnet to run. At current power rates, I doubt if you would even think about it. We built this nice K0 spectrometer, with a bunch of spark chambers to measure the position of particles that were then bent in the magnet, and you measure the position again, and then you got the momentum; and it had an array of scintillation counters that we used to make a trigger. For the first 18 months or so we built the spectrometer. We had a small, but a very good, group of people. Mel's approach to these things was kind of hands-off, because as I told you, since we were actually doing it, that wasn't his primary interest any longer; his primary interest was always the next thing. The first experiment we did was to improve the measurement of the charge asymmetry in Km3 decay. We did a nice job on that.
There's an interesting sidelight, which involves Jack Steinberger, who was Mel's thesis advisor and who had a related experiment at Brookhaven to measure the same charge asymmetry, but in Ke3 decay—that's p-e-n as opposed to p-m-n. Jack also had another experiment at CERN. He was moving to CERN from Columbia at that point, so he had another similar experiment, which was both a KL and KS beam where he could look at both the long-lived and the short-lived kaons. We were competitors, but friendly competitors. There was a workshop at Brookhaven, it must have been in late '72. We had our results, Jack had his results, a lot of other people had other related results. What was unusual about this was that it was an “informal workshop;” the ground rules were to let it all hang out. Show everything you've got; there'll be no proceedings. There will be no quoting of results, so we can have a fully candid discussion. It was a great idea. So, we showed our results, which were pretty solid. Jack showed his results. He had the Ke3 measurement, which basically agreed with ours, but he had a lot more, because he had the KS beam, so he could measure other things. He could measure the KS lifetime. He could measure the regeneration parameter. And he could measure h+-, that involves the decay of the KL into p+p-. None of his new results agreed with the Particle Data Group values, by a lot. It was just amazing. But Jack Steinberger was Jack Steinberger. So, there was an extremely interesting public conversation between Mel and Jack.
You heard it directly, or you heard it secondhand?
No, I was there. Mel would ask a really good question, which is what he did, and Jack would just lob it back. And Mel would ask another really sharp question, and Jack had an answer for everything. It was really interesting to watch. In any case, Jack’s results held; they were correct; all the older measured values had to change. The interesting thing was that we were doing a second experiment with the K0 Spectrometer, with a slightly different group. Mel was not a participant. It was a collaboration between some of us at SLAC, and the University of Colorado group of Uriel Nauenberg. The apparatus was similar, but what we were looking for was the decay, KL to p+p--p0, a 3-body decay; we were looking at the asymmetry on the Dalitz plot, which is again related to symmetry violations. We had already taken all the data. I realized after the Brookhaven workshop that we could also measure h+- with this data. h+- is the ratio of two branching ratios involving the KS and KL, but the experiment measures one of them and then divides it by another. So, I went to Bob Messner, the graduate student from Colorado who was doing the Dalitz plot analysis for his thesis. and said, "Hey, Bob, why don't we try to find the two-pion decay without a p0?" And he said, "Oh, I already did that, and I got this really large value. I thought I must have been doing something wrong, so I just put it on the shelf, and I thought I'd get back to it later." The answer he got was in fact Jack Steinberger's result.
So, we plunged in; we went over the analysis very carefully and decided it was right. Then we had a problem; we wanted to publish, but the ground rules of the workshop forbid publication; no referencing of anything that went on at that meeting. We couldn't publish without somehow acknowledging that we had learned about Jack Steinberger’s measurement. The solution was for Mel to call Jack, who said, "It's okay. We're not ready to publish yet, but just put a footnote in that references a private communication from this workshop, and that would be adequate." We did that and we published the new result. The interesting thing is that this was not a subtle effect. The standard PDG value for h+- was 1.96 x 10-3. The value we got was 2.24 x 10-3. And h+- is the square root of the ratio of two branching ratios. This means that the new measured branching ratio was different by more than twenty percent, an unheard-of change. But indeed, when you measured it with a really well understood apparatus, as Jack did and we did, you got a very different answer.
When did you join the faculty for SLAC? When were you appointed an assistant professor?
That's complicated. There was a two-step process, and you have to know some background. At Stanford, the forerunners of SLAC were a set of electron linear accelerators at the High Energy Physics Lab (HEPL) on the campus, accelerators that Robert Hofstadter used for his Nobel Prize-winning electron scattering experiments. The largest one (the Mark III) had an energy of 1 GeV. When it was proposed to build the SLAC linac at 18 GeV, it was clear it was too big for a university campus; it needed to be housed in a national lab. There was an excellent site on Stanford land, but the relation of such a laboratory to the university was, as I said, difficult to work out.
After SLAC was established, people on campus continued working on linear accelerators, and they wanted to build a superconducting linear accelerator. The advantage of a superconducting accelerator is it can be CW, continuous wave, whereas normal conducting electron accelerators have to be pulsed: the electrons come in a short pulse, and then there's a big gap, and another short pulse. That time structure has a lot of disadvantages. If you can make a superconducting linac, you can do a whole lot of coincidence experiments and things you can't do with a normal-conducting machine. They did some development of superconducting cavities on the campus and got a project started from the National Science Foundation to build a 2 GeV superconducting linac, right on the campus in the same enclosure and hall that they had for the previous linear accelerators years before. So, I started talking to people on campus about making pion and muon beams using the CW electron beam. We knew you could do that; SLAC made plenty of pions and muons at much higher energy. I started looking into whether you could you make intense pion and muon beams of much lower energy, stop them in a target and do muonic X-rays and a lot of other stopped muon experiments or pion experiments. I did some work on that, and actually published a paper in Nuclear Instruments and Methods with an undergraduate at Stanford, named Daryl Dibitonto, about building a superconducting muon channel. It was related to things that people had done for medical purposes to make stopped pion beams for cancer therapy. In fact, something very similar was eventually built at the PSI Laboratory in Switzerland; I actually saw it years later. In any case, this work was sufficiently interesting that Mel worked to get me an Assistant Professor position at Stanford.
Stanford Physics, not SLAC, you mean.
The Stanford Physics Department, not SLAC. Very different. I got this job with a rather peculiar proviso, because there was a fight between Bob Hofstadter and Mel about who would control this appointment. In the end they made a compromise: Mel would make the appointment for the first three years, and then that person would be out on his ear, and Hofstadter would get to make the appointment for the ensuing three years. I got the first appointment, and then three years later, Alan Litke got the appointment.
And you knew this, obviously, going in.
I knew it going in. I had other job offers. I had a job offer from Stony Brook; I also had a job offer from Johns Hopkins for an Associate Professor position without tenure.
So, it was tenure track, but in reality, it was a dead end.
That's right. I've always lived a little dangerously, so I decided I would take my chances. I got the Stanford job, and I worked on the stopping muon experiments. I had an office on campus and was teaching at Stanford, but I spent almost all my time at SLAC, continuing to work on the K0 experiments.
When I first walked into the Physics Department, I went to see the chairman, Walter Meyerhof, as one does when you're starting out. Meyerhof handed me a big red manilla envelope, the kind you tie with a shoelace, and said, "Take this home. Read it tonight. Do not copy it, and bring it back in the morning," which I did. In the file were documents on the establishment of SLAC, which was an incredibly controversial subject. I think a lot of people know that. People like Felix Bloch and Hofstadter were very much against establishing SLAC as a national laboratory with a university connection, and people like Sid Drell and Panofsky, both of whom were on the Stanford faculty, were for it. So, there was just a huge fight about how this would work. The sticking point basically was that Panofsky wanted senior people to have the title of Professor, and be part of the faculty senate, and be full members of the Stanford community. That was a very wise decision. It was how he was able to attract all those Nobel Prize winners-to-be. But it was, of course, incredibly controversial. And certainly, in that original generation, there was a lot of sensitivity. I even once heard Bob Hofstadter say, “Those people at SLAC are not professors; they are not allowed to profess.” That is, SLAC faculty did not teach courses on the campus. That changed much later on.
And you think people like Bloch, their concern was that it would dilute the caché of being a physics professor at Stanford?
I don't know what was in his mind. I think they were worried that SLAC was going to become much bigger than the physics department. That SLAC would just overwhelm the physics department. SLAC became quite large, but it never overwhelmed the physics department, which was quite capable of standing on its own and earning its own Nobel Prizes.
But also, SLAC—I mean, this is much later on, but it branched out from physics also.
I'm not sure what you mean.
I mean, as SLAC evolved, it took on initiatives that were not strictly physics.
It was physics, but not high energy physics. That was much later when the local high energy physics accelerator was closed. We'll come to that later, I'm sure. In any case, I read the folder and I learned a lot.
What do you think the chair's intentions were in just giving this to you?
That I should be informed because I would be dealing with these people, and I should know where they were coming from. That was quite a reasonable thing to do, and very enlightening. At that point, I was the only SLAC postdoc who had become a Stanford faculty member.
What were your responsibilities? How was that worked out from the beginning in terms of how much you would teach, how much you were expected to be doing research at SLAC?
Nobody ever gave me direction. I just did my thing. I taught one course each semester, that's standard, and continued whatever research I was doing. Most of the work I was doing was at SLAC. I did more work on the campus HEPL muon channel and a pion channel, but that never came to anything because the superconducting linac never worked. They changed the RF to a higher frequency, using a smaller accelerating cavity, to make the machine less expensive. That was a disastrous decision, because it brought them into a whole new realm of breakdowns and instabilities. They struggled for ten years or so, but they never made an accelerator that worked. They made one small section—I think it was 80 MeV; the original goal was 2 GeV. Years later, NSF closed it down. I designed muon and pion channels that could be built if there was an accelerator, but there was never an accelerator. Most of my research was at SLAC, but I was fully part of the Stanford community, and did my share of committees, arranging seminars and things of this sort. I'll tell you a colloquium story. One of the colloquia I arranged was for Fred Reines, from Irvine, who discovered the neutrino and did a lot of work with reactor neutrinos. I was taking him around to talk to various people, as is the custom. We opened a hallway door, and coming the other way was Joe Weber, who was famous for his gravitational wave antenna bar that claimed to see gravitational waves but didn't. He was visiting Stanford because Bill Fairbanks was working on a superconducting version with squid readout of the aluminum bar antenna. They saw each other, and it was amazing: Hugs all around; "Freddy!" "Joey!" It turned out that they grew up together in Hoboken.
I can also tell you another story that involves Fred Reines. When weak neutral currents were discovered around '72-'73 in the Gargamelle bubble chamber at CERN, and then later at Fermilab, some of us had the idea that you could use what we knew about nuclear physics to measure something new about weak neutral currents. That is, there is a process called Coulomb excitation, whereby you bring a charged particle, typically a proton or heavy ion, not to collide with a nucleus, but to pass close to it. The passing electric field can excite the nucleus, and you can measure the de-excitation gamma rays. Our idea was if there were weak neutral currents, you could have the analog of Coulomb excitation, where not a photon, but a Z boson was responsible for exciting a nucleus when a neutrino passed by. You could then measure the de-excitation photon. The reason that's interesting was there were certain selection rules involving the quantum numbers of the excited states of these nuclei, so you could learn about the spin and isospin characteristics of the weak neutral current.
We got two theorists from Stanford, Dirk Walecka and Tom Donnelly, to do some calculations with us, and we figured out how to do these experiments. There were several options. One uses higher energy neutrinos with carbon-12 as a target. Another case, with lower energy reactor neutrinos, was fluorine-19. It turned out that there was a scintillator, barium fluoride, that had been recently discovered. It was similar to the sodium iodide that I mentioned earlier as the first muonic X-ray detectors. Traversing particles produce light, you look at the light with a phototube. The idea was to make a target out of barium fluoride, and then if any of the fluorine atoms were excited by the reactor neutrinos you would see the de-excitation gamma rays. The target and the detector were the same, which is an elegant idea.
I worked all this out in some detail, and I read all of Reines's papers, because he had done the reactor neutrino experiments. And I kept coming to a dead end. No matter which paper I read, there was always something missing. In other words, to get the neutrino flux, which I needed to know to calculate the rate in our experiment, I needed to know how far his detector was from the reactor core and the power of the reactor. No matter which paper I read, there was always something missing. I went over and over it. Finally, I went to Mel, and I said, "Let's go over this together, and tell me what I'm missing." I wasn't missing anything. Mel then called Fred Reines and explained the problem. Fred said, "Oh, you noticed." The problem was that Reines worked at the Savannah River reactor in Georgia. That's a reactor used to produce tritium, and the power of that reactor was therefore classified. His deal with the AEC was that he would never put anything into his paper that would allow one to work backwards to calculate the power of the reactor. That's why I couldn't find it. Reines said, "Well, if you have a security clearance, I can tell you, but if you don't, I can't."
So, Mel went to Sid Drell. At that time, Panofsky was on leave at CERN working on an early version of the g-2 experiment and Sid was Acting Director, and after a lot of pulling and tugging, he agreed that Mel and I could get security clearance. The reason there was any resistance was the cost, at that time something like $10,000 each to get security clearance. It must be $100,000 now, but it was $10,000 then. The FBI came around and interviewed our colleagues and my neighbors, who told them, "He comes and goes at strange hours," and things like that, which was true. Nonetheless, we got the security clearance. So we went down to Irvine, had lunch with Fred, and he gave us the missing information. That's all we needed to know. We then could fill in the blanks and figure out whether the experiment was viable or not. The answer was that it was marginally viable. It was one of those situations where if anything went wrong, you'd be in the soup. We decided it was too risky to undertake the experiment. So, we just published a paper in Physics Letters on the concept, but we never actually did the experiment. It took about another twenty years before somebody actually did the carbon-12 version of the experiment using the CARMEN accelerator in Britain, and they were able to demonstrate the effect. They didn't cite our paper, which I always found slightly annoying.
There was another interesting aspect of the original K0 Spectrometer data. One of the features of the beam was that it had a means of ascertaining the momentum of the neutral kaon. That was a very unusual feature. Most kaon beams had an unknown momentum. We took advantage of the very poor time structure of the SLAC beam to have an initial start time when the electron beam hit the target, and produced the KL beam, we got a signal. So, that was a start signal, and then when it came to our apparatus, we had another signal. We could measure that time, and therefore measure the momentum of the kaon. That was very unusual, and it was important. We had a large sample of KL decays that we needed to measure the very small charge asymmetry, of the order of a few time 10-3, and I realized that we had a unique situation. We had measured both decay particles, the pion and the muon. We didn't have the neutrino. But because we knew the momentum of the incoming kaon, which nobody else had, we could do a full kinematic fit. We had an over-constrained situation, so we could place events unambiguously on the Dalitz plot, which nobody else had ever been able to do. The problem was, there had been a whole series of small experiments, some counter experiments, some bubble chamber experiments, that measure the scalar form factor in Km3 decay. All of the experiments had very limited statistics. 10 events, 50 events, things like that. They all had this kinematic ambiguity, so they were not very good experiments. The problem was the answer they were getting was at variance with the hot new theory at the time; current algebra was becoming a dominant approach in field theory. Current algebra made some very definite predictions about the values of these form factors, and the experiments just did not agree with them. I realized that we had a data sample, taken for another purpose, that was hundreds of times larger, and could also resolve the kinematic ambiguity. So, I did that analysis, and I got an answer that agreed with current algebra. Now, that's a very uncomfortable place to be because there's a dozen experiments that don't agree with the theory, and now here's one that does.
Why uncomfortable? Isn't it exciting?
Yes, but the question is, are you correct? Other people aren't stupid. Nonetheless, after going over it and over it, I decided I couldn't do any better, so we published it. It was right. It was the dominant experiment in the field for 20-25 years, and it put current algebra on the map. That distinguished me from the herd a little bit.
Dave, tell me about the E92 experiment.
E92 was based on an idea that I had with Bob Morse from Colorado while we were working together on the kaon to three pion experiment. Bubble chambers had been a big thing in physics, and people built bigger and bigger chambers. But one problem with bubble chambers was that they had no discrimination. You put in a beam, you took a picture, and then you sorted through hundreds of thousands of pictures to look for the few pictures you wanted. There were very ingenious automated devices to help with the scanning, but it was still a very cumbersome process. People were starting to think about triggering bubble chambers. In other words, having equipment surrounding a bubble chamber that gave you information about what had happened, so you would know whether you wanted to take the picture or not. SLAC had a bubble chamber group that ran two big bubble chambers. One was an 82-inch bubble chamber that had originally been built as a 72-inch chamber at Berkeley and was then moved to SLAC because SLAC had higher energy beams than the Berkeley Bevatron. SLAC had also built a 30-inch bubble chamber. There was an electron scattering experiment, done by Elliott Bloom, who at that time was a member of Dick Taylor's Group A, that was a kind of a triggered bubble chamber experiment using the 30-inch bubble chamber. Our idea to do something more sophisticated. The SLAC group was building an experimental 12-inch bubble chamber that was driven electrically, like a loudspeaker, as opposed to the usual hydraulic piston, so it could actually pulse much faster. They had to solve a whole lot of problems because it takes a certain time for bubbles to dissolve, but they thought they could build a bubble chamber that pulsed at a rate as high as 90 hertz. They rarely got up to 90 hertz, but they got close. They had built it originally just as a research project.
So, I said, "Why don't we take this chamber and put it in front of the K0 Spectrometer?" We could then do this E92 experiment with known kaon momenta, which was crucial to what we wanted to do. We had a beam with known kaon momentum, and we had the rest of the apparatus that could measure the outgoing momenta very well, and we had trigger counters. The idea was to put the bubble chamber where there had been a decay space for the KL. We had to build some very unusual multi-wire proportional chambers that were curved. They had wires that ran in only one direction, vertically, in the front and in the back of the bubble chamber volume. These were built at Colorado, and we had to put them inside the vacuum chamber of the bubble chamber, because they had to be as close as possible to the hydrogen volume. They had long cables to take the signals from the chamber out through the vacuum chamber, which made for all kinds of problems: we had intermittent oscillations. You would be running and, all of a sudden, things would go haywire. We never figured out how to solve this, but if you shook the cables the oscillation would stop. And then you would resume the run and the next day it would happen again, and then you'd shake the cables again. That's one of the mysteries of experimental physics, but we got through this, and we did the experiment.
We didn't get quite the statistics that we hoped to because the rapid-cycling bubble chamber kept breaking down. We didn't quite get the efficiency we expected either. Nonetheless, we did an experiment that dominated the field. The interesting thing was that in the beam next to ours, David Leith's group was doing the same experiment, but untriggered, in the larger 30-inch bubble chamber. So we were in direct competition; we wound up with better statistics, since we could trigger the chamber. We still had to scan the pictures, but the scanning procedure was far less onerous than with a conventional bubble chamber. We got two theses from that experiment. One of them was by Jonathan Dorfan, who was from Irvine. I knew his brother David very well; he was a grad student at Columbia. He then joined Mel Schwartz's group at SLAC and became a professor at UC Santa Cruz. David and I shared my first office at SLAC. His brother joined E92 and we've been friends and colleagues ever since and have worked together a lot. Jonathan eventually became Burt Richter’s successor as SLAC Director.
Now, did you know with this essentially dead-end faculty appointment that SLAC was ready to take you on a full-time basis on 1975? Did you have that security in any way?
No, not at all.
Were you on the job market? Did you apply elsewhere?
I went back on the job market. I got an offer from Harvard as an untenured Associate Professor. Harvard was very attractive, but their record of promoting young people was quite poor. I was ready to do that, but one day I walked into Panofsky's office, and I said, "I have this Harvard offer, but I'd really love to stay here. Is it possible to move from Stanford to SLAC?" And he said, "Well, I'll take it up with the faculty." So, they had a faculty meeting and they decided yes. No national searches, no nothing. They just decided yes, and I got that job. I just moved over laterally. So, the only real change in my life was that I wasn't teaching anymore.
Probably an easy decision, all in all.
I was happy with it.
And was the plan immediately to join Burt Richter's group?
Yes. I figured if I was to have any chance of catching on, I needed to be part of a regular SLAC group. That's number one. Number two, in November 1974, there had been the November Resolution.
Revolution; the entire world changed. Everything everybody else had been doing was no longer that interesting, and e+e- was interesting.
What was the status of SPEAR at that point?
SPEAR was running. They had—
Starting with Mark II?
No. The so-called Magnetic Detector, which in retrospect is called the Mark I. The Magnetic Detector sparked the revolution by discovering the Ψ. Then Burt thought about building the Mark II. What was really going on at SLAC was preparation for the PEP collider. They wanted to build a larger 30 GeV storage ring, so they had summer studies at Berkeley, because it was a SLAC/LBL project, in '74 and '75, just like that Los Alamos study in '68, to gin up interest in it. SLAC did not have a very good reputation for being accommodating to outsiders. A well-earned reputation, by the way. I remember an example in a neighboring beam line in End Station B. The group was from UC Santa Barbara, Dave Caldwell's group. Sometimes you would be able to get a running time extension if you needed one. Rarely, but you could at least request it if you needed some more time. David Leith, whose nominal beam line this was, had a crane with a big concrete block poised when the beam went off at 8 o'clock, Monday morning. At 8:01, he had this concrete block lifted right into the beam to block it, so that these guys weren't going to get any more time. SLAC was not terribly accommodating to outside people. So, in order to build PEP—you have to understand that SPEAR was never an approved experiment; I think that's well-known. They kept applying to build SPEAR, and DOE kept saying no. So, what they finally did—
What was the basis for that? What was DOE's objection?
I don't know. Not enough money, or not interesting enough. But Burt Richter had been wanting to build a colliding e+e- beam machine ever since he was at Stanford, where he had worked on colliding electron beams. Electrons colliding with electrons because they couldn't make positrons, but they could at least demonstrate how to make such a machine. He joined the SLAC faculty with the express goal of doing that. He didn't have an apparatus to do it, so he did photoproduction experiments in End Station A for many years while he was trying to get SPEAR built. Eventually what happened was Pief decided to tax each of the groups by some amount to gather a fund to build SPEAR as an experiment. It was never a DOE-approved accelerator project.
I did not know that. That's interesting.
The lab started to work on the approval of PEP before the November Revolution. After the November Revolution, it was clearly going to happen. They knew that they had to be more open to the outside world. So, they had two summer studies, and they formed an outside Scientific Policy Committee which was initially chaired by Karl Strauch of Harvard. And they solicited proposals from the outside community as well as from the SLAC groups, for experiments at PEP. PEP had six interaction regions, so they could have simultaneous experiments. Burt, clever as he was, decided that he would start building an improved version of the Magnetic Detector, the apparatus that became the Mark II, for SPEAR. And then he would say, "Oh, by the way, we have this new really good detector at SPEAR. Why don't we just move it to PEP? We can just move it down the road?” The other SLAC groups put in proposals and didn't fair very well. Taylor put in a proposal that was turned down. Leith did not because he was building LASS. Burt and Martin Perl did, but they had a detector. Moseley did, it was turned down. Joe Ballam put in a proposal to look for free quarks. It was turned down. Ritson, who was not SLAC, but Stanford, put in the MAC proposal that was approved. So, the SLAC faculty didn't fare very well in the PEP competition. There were some repercussions to that and some things that happened later, but Burt was sitting pretty because he had the Mark II.
What were the considerations with you and Gary Feldman being named co-spokespersons?
Gary worked in Martin Perl’s Group E, which collaborated with Richter’s Group C. I should back up and tell you about something that happened with Gary and I at one of those PEP summer studies. In the '74 summer study, this was before the November Revolution, the mystery was that a quality called R, the ratio of the hadronic cross section to the point cross section, was rising with the center mass energy. That wasn't supposed to happen; R was supposed to be flat with energy. Burt (and lots of others) got very excited about that. I remember I was on an airplane with him going down to a conference in Irvine in December '73. This is before I joined his group. Fred Reines used to have little conferences every December. I happened to be on the same plane with Burt, sitting next to him, and he was going on and on about how he had known since he was a graduate student that the electron had a hadronic core, and that was why the cross section was rising. That did not turn out to be the answer, but it was his explanation at the time. So, the problem in the summer of '74 was, why was this ratio rising? I had a really good idea, and I got together with Gary to work it out. The idea was to build a very simple experiment that measured the hadronic cross section entirely calorimetrically. The detector had no magnetic field and no tracker. It just measured the total energy produced in each collision. This energy is completely characteristic. If it's a beam gas interaction, it's half the energy. If it's a cosmic ray, it's even smaller. If it's a Bhabha scattering event, it's smaller and localized. If it's what you're looking for, hadronic final states, it's the highest energy deposition in the calorimeter. So, you could build this simple apparatus, using 12"x12" box girders, fill them with steel plates coated with Teflon, add liquid scintillator, put phototubes on the ends, and pile them up like logs around the interaction region. And in a very short time, you would have a definitive answer, another totally independent method to measure the hadronic annihilation cross section. Gary and I put this in as a SPEAR proposal, and a lot of other people put in proposals, too. As you can imagine, by the time this was going on, it was a hot topic at SLAC. There was a proposal from LBL to improve the Magnetic Detector by building the Lead Glass Wall. That was, to take away one side of the MD that had electromagnetic shower counters that didn't work very well—that's a long story which I'll come back to later—and replace them with lead glass counters to measure photons very well. There was a proposal from Elliott Bloom to do the Crystal Palace, which eventually evolved into the Crystal Ball experiment. There was an experiment from Wisconsin to measure weak-electromagnetic interference in di-muon production. These were a whole bunch of heavy hitters, and then there was Gary and me. The night before the meeting, I got a phone call from Joe Ballam asking me to withdraw our proposal. Not because it was bad, but because the situation was too complicated.
That's how he explained it to you.
Yes, it was a very good idea. But it was two small fry in a big pond with these big whales, and they couldn't approve us and disappoint the big players. So, rather than turn it down, he asked me to withdraw it.
What was your response?
I withdrew it. I had plenty of other things to do, but I was certainly disappointed because it was a good idea and would have solved the problem. But it would have gotten on the air after the Ψ had been discovered, which answered the question: the rising R ratio was due to Ψ and charm production, so nobody would have cared.
Back to your question. Gary and I were two assistant professors in the Mark II group, along with Harvey Lynch and Roy Schwitters. The way Burt parsed things out was that the day-to-day operations of the experiment were in the hands of the younger people. So, he gave us the opportunity to be the spokesmen of the experiment. There was no doubt that Burt was the boss even though we were the spokesmen, but that was okay.
And where does Mark III enter the picture?
Let's do Mark II before we do Mark III.
The Mark I detector was a very forward-looking device. It was the first large 4p detector, and it did great physics. But there were parts where it didn't work very well, in particular the electromagnetic shower counters, which were the responsibility of Berkeley. They used alternating levels of lead and plastic scintillator, viewed by phototubes. As they were building them, they cleaned the plastic with KimWipes, basically tissues, except those tissues have glass fibers embedded in them for strength. This produced microscopic scratches on the plastic scintillator, such that the decay length of the light became very short. If light was produced in the middle, it was very greatly reduced by the time it reached the photodetector on the end. So, there wasn't very much light out of these things, and they performed poorly. That was the motivation for doing the Lead Glass Wall, to take off a side of the MD and put in good photon detection.
Now Burt was going to build the Mark II, the bigger, better detector for SPEAR, but really for PEP. I joined Burt’s group right about when people were thinking about getting started on this. So, I looked around for someplace where I could contribute, and I became aware of a new type of detector that was being built at CERN by a group led by Bill Willis. He was at Yale, then went to CERN, then to Columbia. The accelerator they were using was the Intersecting Storage Rings (ISR) at that time, and he had built a lead-liquid argon detector, which covered only an octant. It had radiators of lead, but the sensitive material was not plastic scintillator. It was cryogenic liquid argon. This detector worked really well; it was a good idea. The device they made was really just jerry-rigged. It didn't have any good insulation when it was brought down to 79 degrees Kelvin; it was just covered with blue Styrofoam. The Styrofoam would crack because of the expansion and contraction with temperature cycling. So, the detector had icicles hanging from it, but it worked. I mentioned this to Burt as an interesting idea for Mark II. He said, "Get on a plane, go to CERN, and learn about it."
So, literally, the next week I went to CERN. Bill was very helpful. They showed me everything about it. I went into enormous detail about all the circuitry for the electronics, and all the ins and outs of what you need to do to keep the argon clean, etc. When I came back, I was very enthusiastic about it, I made a preliminary design, and I ran into some opposition. Not for the design, but for the fact that the Berkeley guys were the shower counter guys. That was their domain, and here I was poaching on it. We fought that out and made an arrangement where we would do a good fraction of the work at Berkeley and some fraction of the work at SLAC, but I would be the system leader. That worked fine. We had our differences, but it worked out nicely, and it produced something quite good; we built this large device that performed fabulously. We had to build a prototype, of course, to prove to people we knew what we were doing. At SLAC we built a cubic foot scale prototype using my design. The LBL people decided they needed their own design, so they made another version. We designed them so that both would fit into the same cryogenic apparatus. We would put one in, run, take it out, and put the other one in, running with the same electronics but a different mechanical design. Mine worked a lot better than theirs, so we adopted that design, and we built it.
Another aside there. I got in real trouble with Panofsky over the prototype. We had a very small group of people, and we were working day and night for weeks in this beam test. We had a very strong cobalt-60 gamma source that we could put beside the dewar to see a signal when there was no beam, just to tune things up. This source had a radioactivity label on it; we kept it in a lead container on the floor against the wall in the area. When the run was over, and we were completely exhausted, we all just went home and went to sleep. One of the janitors found this bright shiny object, which was not in the lead container, and put it in his pocket. He then, after a few hours, thought maybe that wasn't a good idea, and he turned it in. So, I was called on the carpet by Panofsky for letting a radioactive source get out into the wild, and really raked over the coals. That's the only time he ever did that, but I deserved it.
We then built the full liquid argon calorimeter and put it in the beam. It worked right away. I had bought a bottle of champagne that I put in the kitchen in the control room at SPEAR with a tag on it that said, "Open when we see a Bhabha peak,”—that is when we could see a full electron peak in the detector. We ran overnight, and at 10 o'clock the next morning, I was drinking champagne, because we could see the peak that quickly. That's how well it worked. It really was good. It was a workhorse. It was used at SPEAR, it then went to PEP intact and worked there, and finally it went to the SLC. For a long time, it held the record for the best energy resolution for a sampling electromagnetic calorimeter in the Particle Data Group review. I think somebody's done better now, but that's 30 years later, so it was very successful. So that was the Mark II, and now we’re up to the Mark III.
And a decision that you faced at this point.
Yes, I had a decision to make. What happened was that Jasper Kirkby, a guy I knew very well because he had also been in Mel's group, put in a proposal. He had already had an experiment called DELCO at SPEAR. DELCO sacrificed other capabilities to have good electron recognition, as a strategy to find charm using the high pT electron signature. He had put in a similar proposal for PEP, with Taylor, which was turned down. So, he revised it, and put it in again for SPEAR. They wanted to build a new version of DELCO at SPEAR. I saw the proposal, and remarked to Burt, "This doesn't really look like the right thing to do at SPEAR going forward." If you're going to run SPEAR, you need to emphasize exclusive state reconstruction. That is, you needed to have a large solid angle and the capability of good tracking of charge particles, good energy resolution for electromagnetic radiation, good particle ID. All the things the Mark II had, except it couldn’t be as Cadillac as the Mark II. I said, "I think there's a way that this can be done much cheaper and still do as good a job. This is the direction to go, not another DELCO-type experiment." Burt read Jasper’s proposal, thought about it, and said, "Why don't you put in a proposal?" It was just me and my idea; it wasn't going to involve anybody in Burt's group, because they were all getting ready for PEP. So, I involved another group at SLAC, Bob Mosley's Group D, which had built a streamer chamber facility that had done some good physics but was kind of at the end of their string of being able to do interesting physics with it.
So, I made common cause with Group D physicists, and we put together the Mark III proposal. There then had to be a comparative evaluation of the two proposals. The SLAC faculty had been so disappointed at what happened with their proposals for PEP, that they had no taste for taking this in front of a program committee. Pief decided this was an internal matter for SPEAR and gave it to the faculty, which then had several meetings. They took a detailed look. I still have a couple of pages of very detailed calculations of momentum resolution that Burt did to evaluate the relative qualities of the two experiments. They decided that Mark III was the way to go. The difficulty was—remember that the DELCO proposal just came out of the blue—there was already a plan to turn off SPEAR for high energy physics. The deal that Panofsky had made with DOE was that when PEP came on, SPEAR hep would be turned off. So, keeping SPEAR running for Mark III was a big problem. The synchrotron radiation group was already preparing to take over. They had run parasitically at SPEAR for several years. The deal that Panofsky had made was to turn off SPEAR for high energy physics to give it full time to synchrotron radiation, and then use those funds for operating PEP. So, he went back to DOE and negotiated a new deal: SPEAR would run half-time for high energy physics and half-time for synchrotron radiation, and they could also run parasitically for synchrotron radiation when we were running for high energy physics as before. So, they basically could run year-round, and we could only run half a year. That was a good deal, and Pief made that happen. The synchrotron radiation folks kept complaining when they were running parasitically, because they always wanted to run at the highest possible energy, and for us, that wasn't where the interesting physics was. But we had control of the beam energy, so they ran, and they did some physics, although they thought they could do better. But they had to wait until it was their turn to control the beam energy.
Now, when do you start thinking about the move to Caltech?
Well, that happened when I didn't get tenure at SLAC. I was a little surprised about that given that I had just had this important experiment approved, and I never got what I would call a satisfactory answer as to what happened, but it happened. I believe they thought they could retain me with a Permanent Staff, rather than a faculty, position. I didn’t go for that, so I hit the market. I got two job offers. One was the University of Washington, which was now a collaborator on Mark III, and that was a tenure offer. The other was Caltech, which was not a collaborator, and it was not a tenure offer. But I looked at the two places and I decided Caltech was a better fit for me. So, that's where I went.
Was it easier or more feasible at Caltech to maintain your work on Mark III?
Perhaps it was a little easier, but I don't think it would have made much difference. The way I handled it for many years, starting in '79 and basically going on until PEP II was turned off in 2008—that entire time, 29 years, religiously on Thursday morning I went to SLAC; if I caught a 6:00 AM plane, I could be there at 8:30 or 9:00. And either on Friday afternoon or Saturday, if something was happening, I would come back home. When I was BABAR spokesman, I went to SLAC four or five days a week. The fact that that plane ride would be shorter than the plane ride to Seattle was helpful, but that wasn't something I paid a lot of attention to at the time.
Was it awkward at all going back to SLAC?
No. I didn't think about it; I had maintained good relations with everybody. I worked with the same people, Mosley’s group, and expanded the collaboration to several other universities. We just picked up and carried on. I'm certainly not at all unhappy that I wound up at Caltech and not SLAC, especially given the way things have evolved at SLAC over time. A university appointment gave me more freedom and the ability to teach, which is fun, at a really great place.
The Mark III had to be done on a shoestring, but had to have really good capabilities, which made for an interesting design challenge. The first thing we decided to do to save money was to reuse the magnet of the original Magnetic Detector. It had steel plates for a flux return, and a normal-conducting aluminum coil. The coil had been taken out and stored in one of the end stations, and the flux return had been disassembled. We were going to test it and reuse it, because that saved a lot of money. I remember I was teaching at a summer school in Hawaii in the summer of '79; I already knew I was going to Caltech in the Fall, but I was still at SLAC. I got a phone call early in the morning from Bob Mosley. I guess he waited until I had some chance of being awake, because of the time difference. He called me with the news that the coil, which had been supported on wooden 4x4 cribbing, had fallen off the cribbing on one side and now had a dent in it. That was a crisis; we didn't know what to do. We decided to measure electrical and cooling integrity of the magnet coil. We did a ball test; the aluminum conductor has a central round hole for cooling water: you drop in a steel ball and make sure it comes out the other end with no obstructions. That was a way to see whether the dent in the conductor had impinged on the hole. It was fine, but in the end, Pief decided it was too big a risk, and that we should build a new coil.
We kept the Mark I steel, but we built a new coil. Everything else had to be built from scratch. We built a very nice drift chamber, a big cylindrical thing, perhaps 6 feet in diameter and 8 feet long. We reused the tooling that had been built for the Mark II drift chamber, which consisted of a cradle that you turn as you go to make the stringing of wires more convenient. The support is a wide leather belt, held together by giant staples which go through both sides of the leather and hold it up. This cradle had been used for the Mark II and then stored. We decided, again to save money, that we would use it for our drift chamber. That was a good decision until very near the end. We had strung all the wires and were testing the chamber when some of the staples pulled out. The belt broke, and one end of the cylindrical detector fell to the ground. We fixed the cradle and put it back, tested it, and it turned out there were a few wires broken, which we were, fortunately, able to repair.
The lesson I take from these episodes is that it's attractive to try to save money by reusing things, but you may or may not save money in the end. But this time we had formed a collaboration which involved Washington, Illinois, Santa Cruz, SLAC and Caltech—a reasonable sized collaboration at the time. Eventually, we added Texas when Joe Izen moved from Illinois to the University of Texas in Arlington. It was good collaboration; it held together for many years. We did a lot of interesting physics. We made, among other things, the standard absolute measurements of charmed meson decay fractions and the first measurement of the ratio of the lifetimes of the charged and neutral D mesons. We never got much credit for the latter because we did it by an indirect method that measured the semileptonic branching ratios and used isospin, but we got the right answer. People really wanted to see the lifetime difference with their own eyes, so the experiment that got the most press measured the actual decay length of the D’s by using silicon detectors. But we had the answer first. We continued with the Mark III for a long time and did a lot of good science. At the start I had, of course, to form a group at Caltech. It consisted of two or three grad students and two or three postdocs. Six or seven or eight people at a given time. Several of these people went on to do quite well in the field. That's actually been an interesting phenomenon. Apparently, the number of opportunities now in fields other than high energy physics, and/or the overall attractiveness of high energy physics have both changed over time. We used to have a very high fraction of the postdocs and graduate students go on in the field. That number of retentions has reduced substantially now. They can get jobs for twice or three times the amount of money in artificial intelligence or big data, or things of that sort, so a lot of them do.
When in the chronology does SLD get going?
That was the next thing I was going to talk about. PEP was the big game in town, but I had nothing to with PEP. I had nothing to do with the Mark II moving there; I was fully involved with the Mark III. Then, Burt got approval to build the SLC, which was a clever version of a linear collider, where instead of clashing head-on beams, you used the time structure and some interesting manipulations to make one beam, hold on to it for a while, make the other beam, let it out, and have them collide. That required new tunnels and a new detector hall. They decided to build a single detector hall. Then, the question is, what detector went in there? Burt, of course, said, "Well, I have the Mark II, and we know how to move it, so why don't we just do that?" With some upgrades, that in fact happened. But some of us thought that the experimental situation at the SLC energies was quite a bit different from that at PEP and SPEAR energies. You could envision a much better detector than Mark II to confront the physics of SLC. So, we got together people at SLAC—Marty Breidenbach, Harvey Lynch, David Leith, Charlie Prescott, and me. A bunch of people from MIT—Jerry Freidman, Henry Kendall, Wit Busza, Lou Osborne and others. We decided to design what we called a grounds-up detector. In other words, if we had a clean sheet of paper, could we do better? Initially, the detector was called GUCCI. That was my coinage, that stood for Grounds Up Correctly Conceived Instrument. My wife actually bought me a Gucci handkerchief, because it was the only thing she could afford in the store, I think. So, it was named GUCCI, and we had GUCCI notes, and all these things. And then people got cold feet; they decided that wasn't a good look, that the name was too upscale. We didn't want to do that, so we thought about another name, and Marty Breidenbach came up with SLD. If you asked what it stood for, he would say, "It's what naturally comes after SLC." If you asked other people, they would say it stood for SLAC Large Detector. When there were a lot of problems with the accelerator, people would say it's SLAC's Last Detector. So, you can take your pick of what it stood for.
Yes. We had a good time building SLD. I was the system manager for the electromagnetic and hadronic shower detectors, again because of my expertise on calorimetry. We built another liquid argon calorimeter. Marty Breidenbach and Charlie Baltay of Columbia were the spokespeople. They didn't have the concept of rotating spokespeople. They were, and I think still are, the spokespeople. We didn't have an election; that's a modern concept which didn't exist back then. There was a problem, of course, that the SLC barely worked for a very long time. There was a rush to get some physics out before CERN swept the table, which it eventually did. So, Mark II stayed in there and did some physics, and in fact, first showed that there were only three light neutrino species. That made the CERN folks angry, because the Mark II scooped them, but it was perfectly legitimate. Eventually, the overwhelming statistical advantage, because of the better performance of LEP, just overwhelmed what Mark II could do. Over time SLC started working somewhat better, and SLD had its turn. There happened to be an ace in the hole for SLD, which was not a central idea in the beginning. It started as peripheral but became the saving grace. A group of people at SLAC had been working on a polarized electron source. SLAC had expertise on polarized electron sources, so we incorporated this into SLD. That's what Charlie Prescott used to measure sin2θW back in the late '70s. They built a new and improved version of the polarized electron source, and we were able then to take data with the SLD using it, reversing the spin of the electron to limit systematics, and measuring a set of quantities, most importantly the forward-backward asymmetry, which was something that LEP couldn't do. So, that came to the rescue, allowing us to get some unique physics out of SLD. Otherwise, SLD was somewhat disappointing in that regard. We built an excellent detector but got only a minimal amount of physics out of it because the accelerator didn't perform very well.
Part 2 – May 25, 2021
Okay, this is David Zierler, Oral Historian for the American Institute of Physics. It is May 25th, 2021. I'm delighted to be back with Professor David G. Hitlin. Dave, it's good to see you again.
Dave, to start, we're going to go back and develop in a little more detail your recollections about the highs and lows of SLD. So, first, let's start with the uranium story. How did that get started?
Okay. A major part of SLD, which by the way, I hope I didn't leave the wrong impression. SLD was a very well-done project. We built a beautiful detector. But its success was limited because of the amount of luminosity that the SLC accelerator was able to deliver. It nonetheless did some really quite important physics that I'll talk about, perhaps, at the end. So, the uranium story. This ties back to the liquid argon calorimeter that I discussed in the context of Mark II. A major part of SLD was a very large calorimeter covering as much of the solid angle as possible. We looked at technical alternatives and decided that the best approach was again a liquid argon device. With the readout geometry being different because the multiplicity in the final states was different, but basically just another liquid argon calorimeter. The difference was that instead of being only an electromagnetic calorimeter as with the Mark II, it also was meant to be a hadronic calorimeter. That is, it was to measure the energy of all of the strongly interacting particles, pions, protons, etc., that were also produced in the interactions. So, we set out to design a calorimeter that had an electromagnetic section, and a hadronic section, making it very deep because you need to have 6, or 7, or 8 strong interaction lengths, which makes for a very physically large structure. The problem in making a device that has uniform response to electromagnetic particles and hadronic particles is that you typically get a different calorimetric response to these different kinds of particles, even though they have the same energy. That's a problem; the response to electrons and pions is not generally identical, which would be the ideal. Back in the '70s when I learned about the liquid argon detector built by Bill Willis at CERN, and then went on to develop that for the Mark II, Bill Willis did another experiment in which he looked at the question of whether you could you make a liquid argon device that would have “compensation.” That is, typically, the e and p responses are not identical, because some of the hadron energy is lost to neutrons and other things that don't put energy back into the sensitive medium, the liquid argon. Willis's solution to this problem was to use depleted uranium as the absorber, the material that creates the showers. The idea there was that the hadrons would break up the uranium nuclei, which have a low fission barrier, and then some of the fission product neutrons would put their energy back into the liquid argon, and if you tuned the various depths and sizes correctly, they would put the right amount of energy back, and you would get an e to p ratio that was close to 1. That sounded like a really good idea.
Dave, on that point, were there any safely or health concerns in using this?
Not really. Uranium metal is a very interesting material. Uranium is pyrophoric, that is, it easily catches fire in air, but only in small amounts. If you have a chunk of it, or a plate of it, it's not a problem at all.
There's no radiation exposure concerns?
It does emit very low-level alpha radiation, but it's with a many thousand-year half-life. It's not a very intense source of radiation. You do have to handle it carefully; you need a permit, but it's not dangerous if properly handled. You could just walk around with it and work with it. The pyrophoric nature is quite interesting. It means that it actually oxidizes very quickly, so if you want to make an electrical connection, you have to grind a clean spot so there's no oxide, and then quickly make the connection. When you're doing that grinding, it's quite spectacular, because the sparks that come off catch fire in the air.
Willis had only a small amount of depleted uranium to make his test calorimeter. He didn't have very much of it, so he made a uranium hexagonal section in the center, and then surrounded it by six other hexagons made of steel, and then put a layer behind that which had all seven hexagons made of steel. So, there was only one central section made of uranium. He put it in a test beam, and he analyzed the data, and he found, lo and behold, he got this energy back into the argon and he got compensation. He got very good e to p ratio. This had been in the literature for a long time, and I knew about it. So, we decided when we built the SLD calorimeter, that we would use uranium.
Many other people had the same idea; it became fashionable to build uranium calorimeters. Five or six large experiments at the same time in the mid '80s wanted to make uranium compensated calorimeters, using this idea. There was our experiment, SLD; there was the D0 experiment at Fermilab; there was ZEUS at DESY in Germany; there was the UA1 Upgrade at CERN, run by Carlo Rubbia who got a Nobel Prize around that time for discovering the W and the Z bosons. Everybody wanted depleted uranium. There's a lot of depleted uranium around. In fact, the Department of Energy actually had huge amounts of it that they were quite happy to get off their hands. The problem was that it was in the form of a powder—uranium sulphate, in big barrels, stored—I don't know where. Somewhere.
The DOE thereupon formed a committee. I was the SLD representative on this committee. We had a bunch of meetings. The idea was to find a way to turn the uranium sulphate into uranium metal, and then roll it into big sheets that could be used for our purposes. We obtained some depleted uranium sheets, and built a prototype calorimeter, largely at Caltech, and put it into the beam at SLAC. We took a lot of data, but we didn't see any compensation. I got a lot of flak for that. “Don't you know how to build a calorimeter? What's the matter with you? Willis could do it. Why couldn't you do it?” That went on for a while.
But it turned out that Willis had made a mistake. The answer to the problem was found by a colleague named Jim Brau of the University of Oregon, who I had previously known quite well because we were both assistant professors at SLAC, neither of whom got tenure. Jim did a Monte Carlo simulation and found the answer. The problem is that if the sensitive material that's going to gather up the fission neutron energy is a large nucleus like argon, it's very hard to put the energy back into the argon. If you have a sensitive material, like a plastic scintillator, which has a lot of hydrogen, a small nucleus, then you can put the energy back. It turned out the only one of all of these proposed large detectors that had a plan to use a plastic scintillator was ZEUS, the DESY one, and they built a very nice uranium calorimeter that worked beautifully and had compensation. But if you use liquid argon as a sensitive medium, it wouldn't work.
In fact, Jim also figured out what the problem was with Willis's experiment. I described to you the structure, that only the central section was uranium; all the rest was iron. So, when Willis’ group analyzed the data, they chose events in which there was a lot of energy in the uranium. By doing that, they biased the result so that what they were seeing was mostly electromagnetic interactions, not hadronic interactions, which fill up the entire device. So, he was choosing a subset of the data that was electromagnetic, and therefore had the desired e to p ratio; it was actually a physics error. When we understood this, we decided we wouldn't bother with uranium, and we made the whole thing out of lead. It was much cheaper, and it didn't work as well as Willis's erroneous theory said it would work, but it worked fine. The other groups that had liquid argon or other materials that didn't have a lot of hydrogen went on and used uranium, and, of course, didn't get compensation.
So, it was a very interesting little detour where we figured out the problem before we actually built it, but other people didn't. In fact, DOE asked me to run a special workshop on compensated calorimetry at Caltech, in which we had people from all over come to discuss this problem. I was very disappointed that Bill Willis himself did not show up to take his medicine; he sent somebody else from his group, which I thought—I have great respect for Bill Willis, but I thought he should have come to get shot at. Instead, he had one of the younger people get shot at. That's the uranium story. We built a very nice detector, but we didn't have a well-functioning accelerator to put it at. What saved our bacon a bit was there was a group that wanted to use polarized electrons to produce the Z bosons at the SLC that joined SLD rather early on in the game. They developed this polarized electron gun for the accelerator, which could give you linearly polarized electrons either forward or backward. You couldn't polarize the positrons, only the electrons, but it was good enough. They were aiming to do a set of subsidiary experiments which turned out to be the main thing that SLD actually was able to produce in physics. The reason was that it was a unique facility. The competitors at CERN, at LEP, who had four detectors, much more data, and basically did all the other physics far better than SLD could, did not have the ability to make a polarized electron beam.
So, we had this unique feature that we exploited to make a number of unique measurements. The main one was the forward-backward asymmetry, with respect to the linear polarization of the electron. We found an answer which was a bit different from the nearly equivalent, but not quite the same measurements that were made at LEP. A lot of effort went into trying to reconcile this, to see were we right, or were they right, or were we both right, and why was there this discrepancy? No problem was ever found with the experiments, so both experimental results stand, and they are not in agreement, to this day, 30-some years later. It remains a mystery in the Standard Model, which may someday be cleared up.
Dave, what theoretical guidance, if any, might help solve this mystery?
There are models that perhaps could make both results correct, if there were for example a Z prime, an additional neutral weak boson, which also comes up in the context of other discrepancies with the Standard Model. So, it's possible to resolve this discrepancy with something beyond the Standard Model. We don't know quite what the right answer is. There may be a solution in the next generation, because people are talking about either building linear colliders or circular colliders that could—they're not primarily meant to, but they could go back into this regime and explore this problem again. That would be 20-30 years from now, but it might still happen. Anyway, the important output of SLD wound up being this forward-backward asymmetry measurement, which was unique. I guess, that's probably about it for SLD, but there was another related matter. That is, while we were building SLD, for example, my group was still working on Mark III. While you're building an experiment, you like to be doing current physics with some other experiment. To get Ph.D. theses, etc., you need data. You need to do physics. So, we always try to overlap these kinds of efforts while you're in these long time scale construction projects, so that you have some physics to do. We were doing that with Mark III.
Then, because of the crisis at SLAC where the accelerator, the SLC, was not working well, they basically had to throw all the resources of the laboratory at the SLC. One of the things they did was temporarily shut down SPEAR until the SLC problems were solved. It was supposed to be shut down for six months, and then it got extended, and extended again. We used to have television monitors that were up in lots of rooms that would tell you the status of the various systems. One screen would tell you the status of accelerators, another would tell you if the central computing system was up or down, etc. They were in many rooms, so I got a friend of mine who had friends in the computer center to add a line to the display that said, for example “179 days since the temporary shutdown of SPEAR.” And then the next day, it would be 180 days, etc. The director of the lab, Burt Richter, at the time, didn't think that was funny, but we kept it up. In fact, SPEAR never came back on. They just didn't have the resources to turn it back on. They were too busy working on the SLC. The question was how our group, without SPEAR, was going to do physics while waiting for the SLD to go on the beamline.
At around that time, in China, at the Institute for High Energy Physics in Beijing, which at that time was a brand-new thing, they were building an accelerator. There was a very interesting high-level competition that reached up to Deng Xiaoping, in fact. American laboratories were giving the Chinese a lot of help in starting up science in various chosen areas. They had four thrusts: physics, biology, other things. In the physics area, and particularly in high energy physics, they were getting advice from Wolfgang Panofsky, the director of SLAC at that time, from people like Sam Ting at MIT, from T.D. Lee at Columbia, and there was a competition. What sort of accelerator should they build if they were going to build an accelerator? Panofsky and T.D. Lee were advocating that they build a storage ring very much like SPEAR, and a detector very much like Mark III. Sam Ting thought they should build a 70 GeV proton accelerator. Not a colliding beam machine, just a proton accelerator. That would have probably been very much a physics dead end. The Chinese chose to build a SPEAR-like machine, BEPC, the Beijing Electron-Positron Collider, and they built the BES detector. The SLAC accelerator people gave them a lot of help. I went there in 1987 to give some lectures on charm physics. We had a summer school to help bring them up to speed on the physics. We gave them a lot of advice on building the detector; they built one that closely resembled the Mark III.
Actually, there's another little story there. BES copied Mark III in almost every detail, and among the things they copied was the design of our electromagnetic calorimeter. I think I mentioned before that we needed for Mark III to have a calorimeter that was as capable, but much less elaborate and expensive, as the liquid argon system of Mark II. We devised a system using layers of lead and sensitive wires in a gas medium to measure the shower energy. We had a very ingenious mechanical engineer named Knut Skarpaas, who designed an elegant construction technique. There was a central aluminum cylinder, and then a set of spacers that went along the top ribs, with holes in them. Then we strung the wires, and then we took plates of lead that had been encased in thin sheets of aluminum, bent them, put them over the ribs, and then used steel banding around to hold it in place. The load was transferred by the bands through the ribs to the central aluminum cylinder. There was a total of twelve layers, built one on top of each another. They had the same design at BES, but they were having trouble mechanically when they tried to band it. The load was not properly being transferred to the central support.
When I went to Beijing in '87 for the summer school, they naturally wanted to show us what they were doing in the construction. We went into the workshop area where they were assembling the calorimeter, and there were all the component parts for it, including the reels of the stainless-steel banding that you would put around the ribs, tighten, and transfer the load. I walked by one of these reels of stainless-steel banding and I ripped my pants. And that, in fact, was the clue to what their problem was. In the U.S., the banding material has smooth edges so that when it's tensioned the bands slid smoothly on the aluminum ribs. You measure the tension, and you clearly see that you're transferring the load. At that time, the quality of the stainless-steel banding in China was crude; the edges, instead of being smooth, were very rough, and that's why I ripped my pants.
You had this epiphany at the time?
Yes. It wasn't a very good pair of pants, so that didn't matter, but it was the answer to the problem. What happened was, as they put the bands around and were tensioning them, the rough edges of the stainless steel were digging into the aluminum rib it was riding on, and so it would tension up to a certain point, and you'd think it was okay, but it really wasn't. It was because it was caught, so then when you tensioned it, it didn't move because it was caught on the aluminum. It happened that our engineer, Knut Skarpaas, was in Japan at this time, inspecting the vessel for the liquid argon system for SLD, which was being made by Kawasaki in Japan. I was able to contact him. I asked him to come to Beijing to check this out. Knut had been the engineer on the Mark III shower counter, so he was able authoritatively to verify that my guess was correct, and that was the problem. What they needed to do was smooth all the rough edges so they could have the stainless-steel slide smoothly on the aluminum, and then they were able to build the shower counter.
Dave, did you get a sense of the overall decision-making structure for physics in China? Who made the decisions, who approved them, who proposed these things?
It was very much top down. Not that that doesn't happen to a certain level in the United States too. I think less now than it used to. In the U.S., for example, if you want a new accelerator or a new experiment you go through a process. You figure out what you want, you write it down, you prove it to your funding agency, your funding agency tries to put it in the budget, you deal with Congress. Eventually, perhaps, you get approved that way. The way it worked in China then was T.D. Lee goes to Deng Xiaoping and says, "I want this," and he says, "Okay." I don't know if it works that way in China anymore, but it did then.
That was the esteem that China held T.D. Lee in, you're saying.
Yeah. I'll give you another, smaller, example. The hotel they put us in when we were at this meeting was not a very good hotel, so the spouses were kind of unhappy, because a not-very-good Chinese hotel in 1987 was really a not-very-good hotel, and we were going to be there for ten days or so. We complained to T.D. Lee, and within a few hours we were moved to a very nice hotel. That's the way it worked.
Now, you say that this BES was very similar to Mark III.
Why was it not just simply redundant, and why would that have not been a block to doing it in the first place?
Well, it was at some level redundant. It's not a bad idea to have more than one experiment measure something because these things are hard, and it's always useful to have a second opinion. Sometimes they confirm things, and sometimes they say, no that wasn't quite right. So, you don't want excessive duplication, but a certain amount is not necessarily a bad thing. The other thing, of course, is SPEAR was turned off for doing this kind of physics. The Beijing machine is still running, even today. It has undergone a bunch of upgrades for the accelerator, they have built a new, more modern detector, and it still runs and does forefront physics. That never would have happened in the U.S., so that was a very good decision, in fact. When SPEAR was “temporarily” turned off, but never came back, we decided to join the Beijing experiment.
At that point, Frank Porter and I had begun working on B physics together—that's jumping ahead a little bit. We decided that we would, together with several other American groups at that time, most of which had been Mark III-related, join the Beijing experiment. They were happy to have us, because we had a lot of analysis expertise, which they did not at the time. We made major contributions to that experiment for several years. At Caltech we built a new kind of luminosity monitor for the BES detector as our contribution. And then we worked together with them for several years, going to Beijing several times a year, running shifts on the experiment, having collaboration meetings. That turned into a very interesting long-term relationship with a lot of the Chinese physicists, which continues to this day. We did a lot of physics there, but the main thing we did was solve an interesting problem that dated back to SPEAR days. If you remember, I told you about the origins of the Mark III because there had been a competing proposal for the DELCO detector at SPEAR. Well, some version of DELCO actually got built, and they made a bunch of measurements. Their forte was having very good electron identification. So, they used that to detect tau leptons. Martin Perl had discovered tau leptons with the Mark I data at SPEAR. That work was carried on by DELCO, which made a measurement of the mass of the tau lepton. By detecting these electrons and determining that they actually were from taus, they then looked at the energy dependence of the production of these particles, and by doing it where—if you don't have enough energy, you can't make them. If you have enough energy, you just make them very easily. There's an intermediate region where tau production is turning on, where the cross section for production is getting higher and higher, and that region is very sensitive to the mass of the particle you're producing. In this way DELCO made a precise measurement of the mass of the tau. The problem is, because the tau is a very simple lepton with no structure, there were very simple relationships between the lifetime of the tau, its branching ratio to semi-leptonic decays, and its mass. And they should all be consistent with one another.
It turned out, the measurement of the branching ration and the other two were incommensurate. The mass did not fit with the other two measurements, which was a problem. Was this a problem with the Standard Model, or is something else going on with the decays? So, we took the opportunity to work on this problem with the Beijing accelerator. That was Frank Porter and my doing, largely his. And we did a very careful study of the onset of the production of the tau with a very innovative technique, a data-driven search. You would take data at a given place, and then see whether you got an event or not. If the answer was yes, you knew you were above tau threshold, so you went down in energy. If the answer was no, then you may have been below threshold, so you went up. And that's a way to optimize your use of accelerator time. Instead of just scanning over the threshold region, you go to the place with the most sensitivity at any given time. So, we did that, and we got an answer which was rather different than the DELCO answer; it was more precise, and actually agreed with the predictions. It brought the three measurements into consonance. There was in fact no problem at all.
Was this a surprise to you, or were you expecting this?
That's an interesting question. The right answer to that question is, I don't know, and I don't care. Seriously. I mean, you want to go into your experiment without assumptions—Francis Bacon said, "Hypotheses non fingo."
So, that's the right way to deal with problems like this. Because of other experiences I had had with other DELCO results, which were also wrong, I guess if I had any prejudice at all, it was that in fact the experiment was not correct. But you just don't bring that to the party. That's not part of what you do. In fact, there's a technique now in common use which we pioneered with BABAR, which I'll tell you about when the time comes, called blinded analysis, where, as you're doing the analysis of the data, you have no idea what answer you're getting. You just do the best job you can, and there's a secret number that you apply to the data; you find out the answer only after you've decided you can't do any better. That's a way to ensure that you bring no bias to what you're doing. Your question was a very important one.
It's interesting, because I've heard it said many times, if the Higgs was not discovered where everybody thought it was, in some ways, that would be an even more interesting finding than its discovery.
That's right. If anybody has a bias going in, it is that they want to discover something new, not tell you that you already knew the answer, and I'm just confirming it. That's a story that we can come back to when we come to BABAR. But the holy grail is to find something new, to break the Standard Model.
Dave, was your sense, foreshadowing to China's ambitions in physics today, that this was sort of an opening gambit in that regard, that this was the beginning of what they were hoping to accomplish long-term?
Yes. I remember at the time, this was the late '80s, early '90s, the SSC in Texas, the famous accelerator that was eventually canceled, was a very big project. It was at the level of—I don't remember the numbers, but of the order of $10 billion. So, while I was in Beijing, I tried to do a little calculation. It was very easy to find what fraction of the U.S. gross national product the SSC was. It was a little harder to figure out what fraction of the Chinese gross national product this little accelerator and detector were, but to the best I could calculate, it was approximately the same fraction.
Yeah, which indicates also how much smaller China's economy was at the time.
Right, and Beijing at the time, and all of China, was a very different world from now. There was very little traffic in Beijing. Most of it was army and Communist Party staff cars. There were trucks at the institute that still had starter handles on the front bumper. There were horse-drawn carts all over the place. You could see the change coming as we worked there over several years. The last time I was in China for BES was '95 or '96, and then I didn't go back until 2005. The difference in that decade was not to be believed. It just wasn't the same place. They had done so much. And in physics, yes, this was their starter project, and they didn't know a lot, but they learned fast, and they got a lot of willing help from the West. They did very, very well. They're now on an equal footing—I can't talk about other fields, but in high energy physics, they're on equal footing with anybody else. They work on experiments at CERN, they work on experiments in other countries. They have their own experiments, in particular Daya Bay a very well-known reactor experiment that measured an important mixing angle in neutrino physics and scooped everybody else. So, they've had a very successful evolution into modern science. They did it with planning and with resources and skill, and they did well.
Dave, let's move on to PEP-II and the e+e- storage ring. What were some of the difficulties in securing approval for this project?
There were many. We have to back up a little bit. B mesons were discovered in 1977 at Fermilab. They were then explored in the early ‘80s at CERN, at the SPS colliding beam machine. They were explored at Cornell, an e+e- machine that had enough energy to produce them at around 10 GeV. And a surprising thing happened, and that is the measurement of the neutral B meson mixing parameter. There are neutral K mesons and neutral B mesons, and they exhibit the phenomenon of mixing, which leads to all kinds of interesting physics. People knew all about how that works in K decays, because they had been studied since the '50s, but B mesons were new, so we were still learning about them. At CERN, they made some measurements of the mixing of B mesons and showed that they did mix.
And then, back at PEP—not PEP-II, but the original PEP—Mark II and MAC measured the lifetime of the B mesons directly, essentially simultaneously, finding that the B lifetime was much longer than had been expected. These two facts taken together got people excited; theorists went to work looking at it, and there was, from 1974, a very important paper by Kobayashi and Maskawa, which eventually got them the Nobel Prize—we'll come back to that—that showed that if there were three generations of quarks, and the B meson was part of the third generation together with the top quark, then you could have CP violation in the decay of B mesons. When that paper came out, nobody thought CP violation in the B system could be measured. But when the information about B mixing and the long B lifetime came out, it began to seem that perhaps CP violation was experimentally accessible.
Then papers by Harald Fritzsch, and by Ikaros Bigi and Tony Sanda, that showed that a particular B decay, into a J/Ψ meson, the Ψ particle that had been discovered at SPEAR, and that Sam Ting discovered and named J, and a K meson, could have very interesting properties. You could have a very large experimental CP asymmetry, given the lifetime and the mixing, and it could cleanly get at the underlying physical theoretical quantities. CP violation was first measured in the K system in 1964 by Fitch and Cronin, and they got the Nobel Prize for that. But relating their measurement back to the fundamental quark mixing parameters was very difficult, because of hadronic uncertainties; the theory doesn't work well with light mesons, but it works much better with heavier mesons, like B mesons. So, what these theorists showed was that if you made this measurement of CP violation in the B system, you'd be able to work your way back without uncertainties as to what was going on at the deepest theoretical level.
This got everybody excited. We needed to figure out how to do this measurement There's a very clever way to do it, which is in e+e-, use the U(4S) resonance, which is a bound state of a b and b quark, analogous to the Ψ” in the charm sector, which is in the 4 GeV region. The U(4S) was in the 10 GeV region. But you need the right kind of accelerator. If you produced the ((4S), which decays to a B meson and a B meson, then you could use the simultaneous measurement of the decay of the one meson and the other to measure CP violation. The trick was you need to distinguish the relative time of the decay of the one versus the other, which turned out to be very difficult. In conventional accelerators, where the beams had equal energies, you were producing the ((4S) at rest, so that the two resulting B decays went only a very short distance on average before decay; the mean flight path is only about 20 microns. We didn't have the technology to measure that distance with any precision.
The answer to that problem came from Pier Oddone, who at the time was the Assistant Director at Berkeley Lab, at LBNL, a very old friend of mine. He said that if the two beams were not of equal energy but rather of unequal energy, and you tune them correctly—in fact, for our case it was 9 GeV and 3.1 GeV, that provides the appropriate center mass energy to make an U(4S), but you make it in motion, along the direction of the higher energy beam. Due to the Lorentz transformation, the length before decay in the laboratory of these mesons produced in motion is much longer. In fact, with that boost, because of that ratio of energies of about 3, the 20-micron distance becomes about 250 microns, a range that you can measure. Not easily, but you can measure it. We knew how to do that. There were lots of workshops about this, and in fact, Pier's idea was surfaced at a workshop that was held at UCLA under the auspices of David Cline from UCLA, who used to run workshops all the time about everything. He had a very good nose for what the hot new physics was, and he would get people together to think about it.
Pier had this idea. The question then was, could you build an accelerator which had unequal beams? Lots of people thought about it and came up with ways of doing it. I have a table in which I gathered all this information for presentations I made in the past. There were, depending on how you count, something over 20 different worldwide proposals about how to build an asymmetric collider. It required not just building a new detector, you also had to build a new accelerator, because you needed a collider with unequal beams, and they didn't exist. It turns out that the ISR at CERN had run for a short time with unequal beams to do a different piece of physics all together, so at least it was proven that that idea could be done, at least with protons, if not with electrons and positrons. In other words, it was not an off-the-wall idea; it was something that you would at least consider.
So, everybody who had access to an accelerator complex got into the game. At SLAC, people—Pier, who had access to accelerator resources at LBNL, put people to work on this. Then, together with Gary Feldman, who was still at SLAC at the time, he came up with the idea of building a small storage ring, and putting it next to PEP. So, the big storage ring would have a high energy beam, the small storage ring, APIARY, would have a low energy beam, and you would collide the two. APIARY was a pun, not an acronym, because it would produce B's/bees. Other people came up with other approaches. There was an Italian design for colliding linacs. There was a similar idea to the SLAC design at DESY, which also had a large storage ring. There was a proposal to build colliding beams in Switzerland. There were important proposals from Cornell, which had done a lot of the e+e- B physics, and sort of owned that field; they wanted to do this too, so they had a proposal. There was a proposal at Jefferson Lab, which had an electron linac. There were proposals at Novosibirsk. There were several proposals from Japan. There were more than 20 different ways of doing this. There were starting to be workshops on the topic. That's basically when I got interested in this. Frank Porter and I had been doing totally different things at Caltech, but we got together to work on this because we knew it was going to be a big deal. The natural place we would have thought to do this would be SLAC. I had a long history of working there, Frank had a long history of working there, so did Pier for that matter, all kinds of other people. But SLAC wasn't interested. Burt Richter, the Director, wanted to build a large linear collider. The SLC was meant to be a prototype for a really big linear collider, a several TeV linear collider. That's where he was headed. It was not going to be on the SLAC site. It was going to have to be somewhere else because it was too big for the SLAC site, but it was going to be basically spearheaded by SLAC as a project. It would be elsewhere in California if they had their druthers. That was their focus, together with all the problems he was having with the SLC. So, SLAC was just not interested. In fact, Burt called it “boutique physics,” which was not a compliment. It was not higher and higher energy exploratory physics. It was looking at fine details of something, and that wasn't really his bag.
Did you make the case that it wasn't boutique physics, that there was bigger science?
Of course. Burt came around eventually, obviously, but that was his initial reaction. So, what could we do? SLAC wasn't interested, but we wanted to move forward, so we put together the Southern California Consortium for a B Factory. It was mainly us and Dave Cline at UCLA, but we gathered Santa Barbara, Riverside and Irvine, a whole bunch of the Southern California universities, to work on this. We had two different ideas. Frank and I wanted to do an asymmetric storage ring design. Cline wanted to do colliding linacs. He was working on doing it on the UCLA site, not on the campus per se, but across Veteran Avenue, where UCLA had ideas about expansion, which has not even happened to this day, but that was his idea. And our idea was to build it either at Caltech, or at Edwards Air Force Base, where JPL had a sub-site where they had tested rocket motors, and explosives. We designed the CITAR collider at Caltech: California Institute of Technology Asymmetric Rings. The idea was it would be underground, basically under the Caltech campus, which really isn't very big as university campuses go, but was the right size. If it was at Edwards Air Force Base, we would have built it on the surface: SPEAR was not in a tunnel. Because SPEAR, as I told you, was never an approved DOE project, it was actually built in a parking lot and shielded by concrete blocks. You could, with larger concrete blocks, do the same thing for a B factory; that was the idea at Edwards Air Force Base.
We worked on this together for more than a year, having monthly meetings and presenting what we learned. These were open meetings; one day, a couple of people from SLAC appeared at the meeting. At one meeting, Jonathan Dorfan appeared. Jonathan was at the time the head of Group C (Burt’s former group) at SLAC. He had been the progenitor and the spokesperson of the Mark II moving to PEP and then to SLC. He eventually became the Director of SLAC succeeding Burt Richter. And he was an old friend of mine, having worked on our E92 experiment with the bubble chamber that was his thesis. After that we worked together a little bit at SPEAR, but after that, we had never collaborated, but always remained in touch. Anyway, he appeared at the meeting, and we talked. Building an accelerator from scratch on a green field site is not simple. We decided to make common cause to see if we can do this at SLAC. That was why Jonathan came to our meeting, because he thought that was a good idea and something we could accomplish. So that was kind of the end of the Southern California Consortium; we decided to see what resources we could gather up at SLAC to think about accelerator design and detector design. At about the same time, there was a Snowmass meeting. These occur every five years or so: the high energy physics community would gather for two or three weeks, typically in Aspen or Snowmass—that's why they were called Snowmass meetings—and break up into organized groups, work on what they thought the future of high energy physics should be and make comparisons. It is always a very interesting process. It's still called Snowmass, but it's not at Snowmass anymore. In the wake of the GSA Las Vegas debacle where they were having questionable parties, DOE started looking askance at those gatherings. So, we don't do Snowmass in the way we used to. It was a good experience because people were there for extended periods of time. You could always go out for a hike and enjoy the environment, but being the sort of serious, crazy bunch of people that high energy physicists are, a lot of work got done, and a lot of interspecies communication happened, because people were in the same place at the same time. It was a very good thing. So, what goes on now has some vestige of that, but it's not quite the same.
In any case, there was such a Snowmass in 1988, and then there was another one in 1990. The SSC was being designed and built, so the thrust was largely to figure out what the rest of the high energy physics program would be. I ran the group that made the comparisons between the different methods of doing a CP violation experiment. It becomes very contentious because experiment A had to be compared with experiment B and experiment C, to see, with a given set of parameters, which one is more sensitive, etc. So, everybody wants to have that be to their advantage, and you have to be a referee so that you make fair comparisons. We did that, and we wrote a comprehensive comparison report. We decided—well, at least, some of us decided—that the optimal way to do the experiment was with an asymmetric colliding beam machine that had both rings in the same tunnel. Both the high energy and the low energy ring would be the same size. That's a slightly unusual idea. It had two thrusts. One was while we were doing this work at Caltech, before there was any SLAC involvement, we were not accelerator experts. Frank Porter had a friend from grad school back at Berkeley named Jeff Tennyson, who was what I would call an itinerant accelerator physicist. He was really good, but he didn't have a permanent job. He would go someplace for a couple of years and work, and when he was no longer interested, he would go somewhere else. The most recent place he had been was Novosibirsk, where they do the same kind of physics that SLAC did. We haven’t talked much about Novosibirsk, but I've had a very long and productive relationship with people there going back to 1977, the first time I went there.
In any case, Tennyson came to Caltech. We raised some private money from a Caltech benefactor named Millard Jacobs to do some of the design work on CITAR. We decided there were two main problems. One was, since this was going to be a very high current machine, there were going to be problems with heating of the beam pipe at the collision point because of the high current image on the beam pipe. So, we used some of the money to hire experts on cooling at JPL, and they produced a nice report which turned out to be quite relevant to this problem. The other was having Jeff Tennyson work with Frank on how to design the collider. They came up with the idea of using equal size rings having a very large number of individual bunches of electrons and positrons and having them collide one with another. If the rings were of equal size, then the bunches in the two rings would have equal spacing. So, bunch 17 always collides with bunch 17, etc. If you recall, the original idea at SLAC was a small ring colliding with the existing big ring. In that case there are more bunches in the big ring than the small ring because the bunch spacing would have to be the same. So, it's not always the same bunch colliding with the same bunch. That turns out to be a big problem. This was figured out in one of the workshops we held in at Caltech. We had people come from all over the world, all the people who were interested in doing this kind of physics. One of the CERN accelerator physicists, Eberhard Kiel, showed that if you had rings of unequal size, you would get a comb filter effect, which would cause instabilities. So, the original APIARY idea at SLAC would not have worked. Nor would the one at DESY. That was a definitive result; you had to have equal size rings.
So, that's the CITAR design, and that's the way we took the project up to SLAC. We would use the PEP tunnel, which already had one ring, that could be used as a high energy ring, and we would have to build another ring of the same size, but capable only of lower energy. It would have smaller magnets, and be cheaper to build, but would have the same diameter. The idea of having a large current made up of a large number of small bunches that didn't each have a lot of electrons in them was not initially well-received. In fact, when we took it up to Berkeley to present the idea, we had sent them a report beforehand, and I'll never forget this. Mike Zisman gave the Berkeley response—this was in the days when we used plastic foils. Mike took the front page of our report, made a foil from it, and then made a bunch of black marks on it that were meant to look like bullet holes; they were shooting holes in everything we had done. But in fact, we were right; it is the approach we used in the end. Their issue was the rapid growth of microwave instabilities, which we dealt with by an extensive feedback system. We worked together on PEP-II with the people from LBL and people from SLAC, and Jonathan even involved people from Livermore Lab who had expertise in RF structures.
So, the eventual proposal was a three-lab consortium, plus a lot of universities, of course, for BABAR. The three labs were responsible for the accelerator. We had to jump through many hoops; there were, of course, all these other proposals going on. Eventually, one by one, they all failed, except for the Japanese collider, which was approved and was our main competitor. Their accelerator was called KEK-B. KEK is the name of their laboratory, so it was called KEK-B for an accelerator that produces B mesons, and their detector was called Belle. So, PEP-II/BABAR and KEK-B/Belle became intense competitors for measuring CP violation in the B meson system. We were the two groups left standing in the end. But in the U.S., we were nowhere near DOE approval—I'm telling you the story up to the early '90s. We were not approved at that point. It still wasn't clear that we were part of the SLAC program at all. We had a bunch of committed people. For the detector, the experimental side, we had maybe 20-25 people. For the accelerator, we had people doing bootleg work on the design. Not many people were really tasked to do it, but people were interested, and they put time in. We were designing technically challenging asymmetric colliding rings, and a detector that could do this physics with asymmetric collisions. So was Cornell, because at that time Cornell was the home of B physics and e+e- in the United States. Cornell is an NSF lab, SLAC is a DOE lab. So, this was not only an inter-laboratory, but an inter-agency competition.
I ran workshop after workshop to get people committed, because you have to build a collaboration; you need a lot of people to do this. The BABAR collaboration eventually grew to about 600 PhDs and grad students from all over the world. So, the competition went on. It wasn't at all clear how this was going to turn out. First, we had to make the B Factory part of the SLAC program. At that point, we made common cause with David Leith, who was the SLAC Director of Research. He controlled of a lot of resources on the experimental side, so that was extremely important. He was also very important in building worldwide interest in collaboration. We worked very closely together to try to make this a part of the SLAC program. I was elected as head of the SLAC User's Organization. Back in the '70s I had been chair of SLUO for two terms So this, with a long hiatus, was the third time. I used this SLUO position to build the support of the laboratory community for PEP-II/BABAR. We ran workshops not for doing B physics per se, but to compare other experiments that could be done at SLAC. One of these was to build what's called a tau/charm factory. That is a collider like SPEAR, or like the Beijing machine, to be built at SLAC. That's a smaller, cheaper thing to do. It would do charm physics and tau physics, but not B physics. There was another experiment to do charm photo production at SLAC. So, we ran SLUO workshops and had discussions about which of these should be the centerpiece of the SLAC program going forward. We worked through that, and eventually it became clear that it should be the B physics program.
When we finally had the lab and community on board, we then faced the external competition. How do you get this into the DOE program? At the same time, Cornell was trying to get its proposal into the NSF program. That went on for quite some time. In the meantime, I was offered the job as the head of the high energy physics group at Argonne National Lab. Argonne, which used to have an accelerator, is a large multipurpose DOE lab not far from Fermilab. It had the ZGS in the '60s and '70s but didn't anymore. It has a very large experimental high energy physics group. They offered me the job of running that division with the idea of positioning the group to be a prominent player in building detectors for the SSC, which was an interesting opportunity. I thought about it very seriously. I went out there, looked at houses, all the things you do. In the end, I made what was regarded at the time as probably a foolish decision, which was to stay at Caltech and to work on getting the B Factory approved. Had I let it go, that doesn't mean it would have failed, but I was certainly a prime mover in what was going on.
Anyway, I decided that I was committed to the B Factory, and I was going to just go for it from Caltech, so I turned down the Argonne job. We needed to find a way into the federal budget. Richter was a master of this sort of business, working with the various congressional offices, with the OMB, etc., [HDG1]to bring this to the fore as a part of the SLAC program. The scale was just under $300 million at that time, not a small number. We organized delegations through SLUO to go to Washington en masse to visit the congressional people in your own district, or around your district. This proved that politics is indeed local. For example, if I went to talk to a congressman from Massachusetts because we had MIT collaborators, their initial reaction would be "Well, why are you here? You're not from my district." I knew that would happen, so the ready answer was, "Digital Equipment Corporation is in your district, and we use Digital Equipment Corporation computers." You always had to have an answer ready, since a question like that was going to come. We walked the halls of Congress, and we talked to everybody. I remember talking to Nancy Pelosi at that time. She had just become a congressperson in the early '90s. Among all the various people, she was one of the most impressive. We also met with Dianne Feinstein in Burt Richter's office at SLAC. She was much less impressive but was helpful nonetheless in getting the approval. We also went to Sacramento seeking state aid, which never panned out. We worked with Carlos Moorhead, the congressman from Pasadena, who was the longest serving member of the California delegation at the time, which means something; seniority means a lot So, it fell to him, for example, to organize a delegation letter; the idea was you wanted to make sure you had the full support of the California House delegation. We did all of this kind of work in support of getting the funding.
It needs to be said that this was all in the context of real threats to the existence of SLAC as a high energy physics laboratory, because the SSC was being built; it was going to be the big game in town for DOE. They eventually made a horrible mess of it, but that's another story. They were becoming concerned with how to find operating funds for the SSC. New money was provided for the construction, but when it comes to operations, you always have to redirect the program to find the operating money. You may get something additional, but most of it is expected to come from the current program. So, how do you do that? You have to stop doing something else, and the favorite candidate was to stop SLAC hep. There was Fermilab, which was the flagship, and was running the Tevatron, and looking for the Higgs and the top quark. So, one idea was to stop doing high energy physics at SLAC. There were committees considering this and we were quite worried that they would recommend that. So, it was very important for SLAC to have a first-class flagship project that could do important physics, and to say, this is our program, this is important, we need to do it. We made some headway but were not getting all the way to approval. Then, the SSC was canceled. That was, as far as I considered, an unbelievably stupid decision. It was a purely political decision on the part of Clinton. They were trying hard to balance the budget, and they were being very stringent about outlays of this kind. They decided that one way to do this in the scientific realm was to kill something, and apparently the two candidates were the Houston Spaceflight Center and the SSC. It came down to the SSC, which I think is extraordinarily unfortunate for U.S. science and for basic knowledge. Everything that's being done now at the LHC, with major U.S. participation, would have been done years and years and years ago, better, had there been an SSC. So that cancellation was just dumb, and the U.S. program has never actually fully recovered. The physics community bears some responsibility, but the DOE bears a great deal of responsibility for mismanagement. It was unbelievably badly done. I was never part of SSC activities, but I was on review committees, so I could see what was happening. It was a bad show. In any case, when the SSC went down, it was a new ballgame in high energy physics. One way to think about it is that PEP-II and BABAR were something of a consolation prize. We'll never know, but it is what happened. So, soon after the SSC cancelation, we were approved.
Are you the main spokesman at the beginning? That's part of the approval, that you would be spokesman?
No. We didn't have a spokesman, per se, in the detector organization, but I was running it. I didn't have a title; I was just doing it. The first thing we did when we were approved was to break out the Aberlour from David Leith's desk drawer. He was Scottish, and when he came to SLAC in the late '60s, he brought along the concept of single malt scotch. He was a great advocate and emissary for single malt scotch. Beltramo's was a large liquor store in Atherton. It's no longer in existence, I think, but it was a very good wine and liquor store. He convinced them to start carrying several varieties of single malt scotch. To this day, I still have many kinds of single malts at home. Later on, maybe, if there's time, I can tell you a story about single malt scotch in his retirement, but let's leave that. Jonathan and David Leith and I toasted our success. And then we went and had cake and champagne with everyone else.
And then there was a problem, whose name was Daniel Moynihan, the powerful Senator from New York, doing constituent service. He put his fingers into the pot, and then we had a new situation. Yes, the B factory was approved. DOE would build the B factory and associated detector, but it could be at SLAC in California, or it could be at Cornell in New York. The money would come from DOE, which would be straightforward if it were built at SLAC, but perhaps it would be built at an NSF lab in Ithaca. So, how did DOE resolve that one? They formed an inter-agency committee run by Stan Kowalski from MIT, a neutral party. This had to be done very quickly because of the exigencies of getting the project into the budget. So, with very short notice, we had a committee proceeding. There were two meetings. One was at SLAC in which the two accelerator concepts were compared, and the other one was at Cornell in which the detector concepts were compared. I basically didn't sleep for about three weeks, the time we had to put together a report and plan the presentations. We had a very small crew of 20-25 people, and everybody worked really hard. I had to put this all together into a coherent document. I thank Steve Jobs and Tom Rokicki, for that. The reason was the Next computer. At the time, Steve Jobs had left Apple, and he had formed another company called Next, which made a really great computer. I had access to one, not at Caltech, but at SLAC. It had a beautiful high-resolution screen, and it had an implementation of TeX, the typesetting language, written by Rokicki, which was far better than any implementation of TeX ever. It was as good as anything that exists even now, in that it allowed you to write text and see it rendered nearly instantaneously, so you would work on a very complicated manuscript very efficiently.
I had to get the various contributions edited, make it all make sense, and put this report together, and then go to these proceedings at Cornell. Both labs’ proposals were perfectly viable, both for the accelerator and the sector. But the approaches at SLAC and at Cornell were very different, and that was one of the arguments we made. The Cornell style was do everything cleverly and cheaply and make something that sort of worked, and then fix it. That had always been Cornell's style. originated by Bob Wilson, who built the Cornell accelerators. He eventually became the first Director of Fermilab, and he built the first Fermilab accelerator in that style, which was not a good idea on the scale of a 200 GeV accelerator, so it didn't work very well at the beginning. They had to spend a lot of money and a lot of time to eventually get it to work. So, that was Cornell's style, and that's the way they would have done the B Factory. And it would have worked eventually. SLAC's style was different. We always emphasized that we were building a factory. A factory runs 24 hours a day; it has to be reliable. It has to work. So, our designs were very carefully engineered, and had some redundancy. All electronics were burned in so that if anything that was going to fail did so before you installed it, etc. It wasn't quite the space program, but that was the approach we took. We said, "We’ll build this. It will work, and we'll get on the air right away." We had competition from Japan because they had also been approved, so we were in a race.
Our argument prevailed. The Kowalski Committee wrote a “on the one hand, on the other hand” report. It just made comparisons; it didn't make any judgments. They basically put it back in the lap of the DOE office of high energy physics, which perhaps not surprisingly decided to do the B Factory at SLAC. So, that's what we did. Senator Moynihan did his best constituent service, but it didn't work. Cornell actually kept running and did a lot of physics while we were building. The luminosity of their existing collider was nowhere near our proposed PEP-II luminosity, which actually led, a little bit later, to a story. In the late '90s, when we were about ready to come on, NSF decided that the Cornell collider would be turned off. They then were able to actually reconfigure their accelerator to run at a lower energy and do some nice charm physics, so they survived years longer. They did that quite successfully, but they went out of the B physics business. So, their group had a party when they were being turned off. I guess it became a sort of rowdy party. They had a total integrated luminosity of 9 inverse femtobarns. That's the unit of measure. So, they took a photo—this is before the iPhone, so somebody must have had a real camera—of two of their collaborators, who I can't identify, mooning us, with “9 inverse fb-1” painted on their buttocks, and with the spokesman of their experiment at the time, holding up a sign that said, "To our friends at SLAC." I was sent a copy of this, which I still have. To motivate the troops at SLAC, as we were working quite intensely, I put this photo up on my office door, which got me into a lot of trouble, because one of the secretaries took great offense. She caused a ruckus and eventually quit over it. They were going out of business, and were saying, this is how much we’ve got, what are you going to do? Well, it turned out that we did very well, and we got triple that amount in our first year, on the way to a lot more and a huge data sample.
By the end of 1994 we were fully approved. The groups building the accelerator had been working together, the three-lab consortium doing design and R&D, for years now. On the experimental side, it was still a small group of people. We needed to expand it dramatically. David Leith and I had been working at that, and had made some sorties around Europe, finding groups that were interested, and which funding agencies would be interested in participating. So, we decided to have a workshop in December 1994, with just a few months' notice. I made a workshop announcement that looked like a formal wedding invitation. You're Invited, … to a workshop at SLAC, and sent it to everybody we knew. Hundreds of people showed up from all over. We used that workshop to start putting together a formal collaboration. We had people from the U.K., France, Italy, Germany, Russia, a few others, eventually Spain, Norway, Taiwan—it became a very large collaboration.
We had to do some rather quick organizational work. First each country's delegation had to organize, and the Europeans had a lot of experience with how to operate in the CERN environment, so they knew how to do this better than the Americans did. They chose national representatives, and we identified US representatives, and we formed an exploratory committee. We decided on an organizational structure. We formed a subcommittee to write a governance document with a set of rules. We had had those kinds of things in other experiments; we weren't starting from scratch. We made a structure that had a spokesman, a deputy, etc. We had to find a title for the collaboration. The way this all worked was to have a nominating committee of senior people with some balance between the U.S., which was about half the collaboration, and one representative from each of these other countries, except for France, which had two funding agencies. That group nominated me as spokesman. We also had a Collaboration Council, with one representative from each institution, to deal with some of the more formal institutional needs, not the administration of the project, or the physics. My nomination was voted on by the Collaboration Council, and I was selected for a three-year term. Three years later, I was renewed for another three-year term. We were still in the middle of construction at that time. That's the way it worked and works now in most collaborations. In the old days, including a whole bunch of things I did, if you founded the experiment, you were the spokesman, until you decided you wanted to do something else. It doesn't work that way anymore. As these things got larger and larger, they became somewhat more democratic. Basically, to gather these resources, you have to give people a voice. So, it's usually clear who the progenitor is, and who makes it go, etc. But eventually people want to have their turn at it, so it's now become an elective game, which I think makes a great deal of sense.
We then had to choose a collaboration name. I had started using the name BABAR back in the late '80s. I did that more or less to make fun of the fact that people made these somewhat tortured acronyms for their experiments. Sometimes it would be as straightforward as CMS for Compact Muon Spectrometer. That's not very elegant, but it actually makes some sense. Sometimes it would be ATLAS, which is another LHC experiment, which chooses letters from a bunch of words, not all of which are the first letter in a word in order to spell it out. I regard that kind of thing as silly. I thought let's have a name for the experiment that's not an acronym but has some relevance. I decided that Babar, the children's book character, sounds a bit like B and Bbar, but it's not an acronym, so let's call it BABAR. I started using clipart of various Babar images, and I started buying first editions of Babar books and Babar toys; I now have a collection. Some people liked the idea, some people absolutely hated it, thinking it wasn't serious. David Leith didn't like it at all; he thought that a new collaboration should choose its own name. I was not happy, but I decided to have an election to choose the name of the collaboration. We chose a committee chaired by John Fry from Liverpool. I was hands off; he ran the election. He asked people to submit names, then he and his daughters sat around on a Sunday afternoon, went through the long list of submitted names, and pared it down to about eight names, including, of course, BABAR. As soon as he told me that, I said to myself “No problem. BABAR will win." Everybody had heard of Babar; nobody had heard of the other choices. It wasn't any sort of sophisticated ranking voting. It was just choose one. So, naturally, a majority—I don't remember if it was a plurality or a majority—went for BABAR, and the rest of the votes were scattered around all the other random names. So, the collaboration name became BABAR. A coda: Klaus Schubert, the German representative on the Executive Board kept insisting, after the fact, that the name had to be an acronym. He came to a meeting with a list of quite tortured acronyms for BABAR but was unable to muster support for the idea.
There you go. Name recognition is valuable in physics as well as politics.
Exactly. I thought it was an amusing name, but Babar is copyrighted. I had been using the logo long before we were an actual project. When we would have a workshop, I would make signs like, "This way to the workshop," and it would have a Babar character on it. One day I got a call from the business office at SLAC asking, "Do you have permission to use the logo?" I told them, "No. You're the business office. See if you can get me permission." They went away and near as I can tell, did exactly nothing. But then they came around again and somebody else said, "Do you have permission?" So, I decided I had to do something. It turned out that the Babar books were originally written in the '30s by Jean de Brunhoff. He wrote about six books before the war for his children, and his wife illustrated them. Then he died quite young, and sometime after the Second World War, his son Laurent wrote many more Babar books; there are now a lot of books, TV cartoons and licensed toys, and all that kind of stuff. It turned out that Laurent de Brunhoff wound up living in the United States and was married to a woman who was a professor of English at Middletown College in Connecticut. I don't remember how I knew that, but I knew that, so I tried to contact him. This was the old days when you could call Information. I called 411 and asked for the phone number and address of Laurent de Brunhoff in Connecticut, and they gave it to me. I wrote him a long letter explaining about B mesons and CP violation and how much we had enjoyed using the Babar logo, and how it would be a great honor if we were to be able to associate Babar with his science, and would it be possible to get his permission for this? I never received a reply, but apparently, he contacted his licensing company, which was called Nelvana—it's now gone on to some other company. They were the licensee for the books and the toys. Nelvana contacted the SLAC business office and gave us a five-year royalty-free license. So, we were all set. The license came with a bunch of, you can do this, and you can do that, with a style manual of how you could depict Babar, which was actually quite amusing. It had rules, like Babar never carries anything in his trunk. And then, you turn the page, and there's a picture of Babar with a tennis racket in his trunk. So, their rules were sort-of-rules, not really rules. In any case, we used Babar, writing the copyright information under any use of an image, and put it on our webpage, and then all the collaborators made their own institutional webpage the same way. We decided, for example, we wanted to make T-shirts, so I had to get permission to make T-shirts. Would that be allowed? They wanted to know how many T-shirts. I said 500, which was fine; it was such a small number that it didn't bother them. They said OK, so we made BABAR T-shirts and other things of that sort. We had a lot of fun using this logo, and the license was renewed several times. I don't know if it's still renewed, even though we're publishing papers, but it did work very nicely.
Our second collaboration meeting was at École Polytechnique outside of Paris in February ‘95. I got there a day early and I went to the Clignancourt flea market in the north of Paris and was browsing through the used books looking for Babar first editions. I found one, and opened it, and inside the cover was what I call a bluebook. The kind used to write exams in college; this cheap set of lined pages stapled together, with a light blue cover. It was all written in beautiful cursive and was about 20 pages long. It was just inside the book, so I opened it up, and it was the Napoleonic Code contract of marriage—France has Code Napoleon, not English law, and in Code Napoleon, to get a marriage license, you must have a contract that says the groom is bringing this to the marriage, the bride is bringing this—an inventory of your piano, and your stove, and your this and that. This was the Contrat de Mariage for Jean de Brunhoff, the original author of the Babar books. I needed this, of course. The owner of the store had put it in the book to just keep it flat and keep it safe. I bought the book and the Contrat de Mariage for, around $75. This was before Euros, so I don't remember the conversion rate, but around $75. We eventually scanned it in, and it's accessible from the BABAR website. The SLAC archivist gave me an archival envelope to store it, so I'm taking good care of it. We had a lot of fun with the Babar stuff, to this day. We should get onto Babar physics.
Let's talk specifically about the connection between CP asymmetries and the 2008 Nobel Prize.
We’ve already discussed the need to have an asymmetric collider. It also had to have a much higher luminosity, intensity, than any other accelerator had ever had. The combination of those two generated quite a bit of skepticism in the community, including with Burt Richter, who was not only a particle physicist, but an accelerator designer. So, he was very skeptical, and part of convincing him that this should be part of the SLAC program was convincing him that we could in fact build A collider with this unprecedented luminosity, which eventually, we did. We had to build this robust B factory, as I said, that in an unprecedented configuration that reached unprecedented intensity. We did that. DOE cooperated by giving us a budget which was basically what we asked for, and giving it to us on a timely schedule, which is not often something that happens. They usually stretch out the funding profile. But they gave us the money we needed in a timely way, and we did our job in a timely way, and we were able to get on the beam line about a month ahead of schedule. We took data quickly; normally when accelerators start, they don't work very well, but they eventually learn how to run it and get better and better. It might even take years to get up to the design intensity. But PEP-II got up to the design intensity in less than a year. The whole thing went off very well. Not that we didn't have our problems. We had plenty of problems. It took a lot of technical ingenuity; it took a lot of financial ingenuity to get around problems and deal with shortfalls. We did all of that. We, in fact, won an award from the DOE for project management, which was interesting. Jonathan Dorfan and I went to Washington to accept this award for how well we had done the project management. We came very close to missing the award, because a big trailer truck that was backing down an incline got stuck and blocked the street. We couldn't go in; we couldn't go out. By the time they got the truck unstuck, we got to the place and ran in as they were announcing the award; we were almost late to our award for being on time. But we got there just on time to accept the award, at which I remember our closest competitor was the re-piping of the National Petroleum Reserve in Texas.
We got the detector built on time, not without a lot of interesting problems that I won't go into. It's a story in itself, but we did it well. We put together a team that worked together very, very well. We were ready to do the physics analysis at t=0 in this blinded way, which I'll come to in a moment, knowing that we were in a race with the Japanese. To exhort the troops, I would always tell them that we're running a sprint and a marathon at the same time. In other words, we want to win the race in the short term, but we want to do things sufficiently well that we have longevity to be able to gather the enormous amount of data to take on the very large smorgasbord of physics that we were capable of doing. The centerpiece, of course, was the CP violation. Now, Kobayashi and Maskawa had given us a range of results that we would get for the CP asymmetry. You couldn't make a precise prediction because the value depended on the mass of the top quark, and the top quark had not been found when this work was all started. So, we used to show a range. It might be here if the top quark is this. It might be there. But eventually, the top quark mass will be known, and it will be a precise prediction. In the late '90s, the top quark mass was measured, and we could zero in on a target value of the Standard Model prediction. That would tell us with some precision about the underlying quantities of the Kobayashi-Maskawa matrix.
Then the question is, how do we do the analysis? The idea of blinded analysis was just starting. In fact, one of the first people to do it was my friend Bruce Winstein, who I'll come to in a little bit, who did a lot of the important work in the later stages for measuring CP violation in the KL system, the other system that shows CP violation. Bruce made measurements, and then at CERN they had measurements, and there was a Standard Model prediction, and things kept going back and forth with experiments not agreeing, getting a different answer than the Standard Model, doing it better, everything changing around. So, for his last and best measurement, he decided to do a blinded analysis. The idea was, as I said, you do the analysis in such a way as you're getting a number, and you refine it, but you don't know the number because you're missing an important quantity which is kept in a sealed envelope someplace by a trusted person. Then, when you finally say, "This is it," you open the envelope, plug in this number, and now you know the answer, so you could not possibly have been biased. In our instance, since there was a Standard Model prediction which was reasonably precise, if the Standard Model was right, we knew the expected answer. The question is, how do we do the blinding, and should we do it at all?
So, we had a meeting, in Padua. I remember standing up in front of our group in a big auditorium and having a free-wheeling discussion about his. Do we do blinding? If we do blinding, how do we do it, because it could get rather complex to keep the number a secret? Do we do this at all? In the middle of the discussion, one of the younger people said, "Well, there's no reason to do a blinded analysis because we know the answer." That made up my mind to do a blinded analysis, because if you're going to come to it with that bias, I want to be sure you don't know the experimental answer, even though you think you know what the answer's going to be. We decided we would do blinded analysis, and we do blinded analyses to this day. There are instances where you can't apply it, but in almost all cases you can, and that's what we do. It's been a very, very successful idea. Our first result was not terribly precise; we refined it as we went along and got more and more data. But it agreed pretty well with the Standard Model. Our Japanese competitors got a similar answer at roughly the same time that didn't agree so well but had very large errors, which kind of covered the discrepancy. So, there was always the question of, who did it first? We did it first. I think even they agree, but so close that it doesn't make any difference. We're both recognized as having done it first, and effectively simultaneously.
So, there were two levels of the exposure of the number. One was the first one, in 2000, at a conference in Osaka, and another was when we had more data a year or so later; we had an effect which was fully statistically significant in 2001. I'll come back to that in a minute. Let me come back to Bruce Winstein for a second. He and I were old friends, and he was on a committee at SLAC called the Scientific Policy Committee, which back in 1989, had to give us an approval, had to say this should be part of the SLAC program going forward. We made a lot of presentations to that end, and he was on the committee. After the meetings there was a dinner associated with these committees. You take people out to dinner and discuss things that had happened during the day, etc. After the dinner, in the parking lot, Bruce and I were talking. He said, "We're going to approve this. We think it's a good idea, but it's going to take a really long time for you to actually make the thing work and get the data, because we really don't believe you're going to come up to speed as fast as you're claiming. The accelerator's not going to work that well." So, I said, "Okay, let’s make a bet." The bet was for a case of very good French wine—Château Lynch-Bages, which is a first-growth Bordeaux, Premier Grand Cru Classé—that we will have a three standard deviation measurement of CP violation in the B meson system by the turn of the century. That's the way we phrased it. I chose the wine. The reason I chose that wine is I had a case of this wine that I bought at a very good price from Trader Joe's because the 1977 vintage was regarded as not a particularly good vintage for Bordeaux wines. A lot of this excellent wine wound up at Trader Joe's, which is not like Trader Joe's now. It used to sell these odd lots of things. It originated in Pasadena, and I actually knew the owner, Joe Colombo. He was a friend of one of my postdocs, RoseMary Baltrusaitis. He used to be a member of the Athenaeum (the faculty club) at Caltech which has members from the surrounding community, not Caltech people. He used to drink in the late afternoon at the bar, as did RoseMary, so I met him through her.
I had this case of wine, which also had a physics connection because the director of Fermilab at that time, John Peoples, was an assistant professor at Columbia when I was there. In fact, when I became an instructor, I shared an office with him. One of his favorite wines was Château Lynch-Bages, the same wine. It was a wine that even a graduate student could afford in the mid '60s, amazingly enough. The b quark was discovered at Fermilab in 1977. So, I had this physics connection to the wine, and I had a case of the wine. I said, "Okay, we'll bet a case of this wine that we make this measurement by that time." I promptly forgot then about the bet. Then in 1999 I got an email from Bob Seimann, a SLAC accelerator physicist, who happened to be standing next to us when we made the bet. He had written it down, so he then, ten years later effectively, exhumed it and wrote an email to Bruce and I saying:
“Eons ago in the parking lot of the Stanford Park Hotel a bet was made between Professor David Hitlin of the California Institute of Technology and Professor Bruce Winstein of the University of Chicago. These esteemed scholars bet One Case of Chateau Lynch-Bages that a three standard deviation signal of CP violation would be seen in B meson decay by the end of the millennium. Professor Hitlin took the affirmative side.”
We were just starting to take data. The question is, would we make it? The next question, of course, is when is the turn of the century? There's a philosophical debate. Is it 2000, or 2001? Do you start counting the Gregorian calendar from 0 or 1? So, we had an interesting—I shouldn't call it philosophical, it was theological—debate about this that was a lot of fun. Several books were written on the subject. In any case, we did not make either deadline. We made the greater than three sigma measurement in 2001, in the summer. A nice enough result for the bet, but it did not meet the turn of the century by either definition, so I owed Bruce a case of this wine. There was a Snowmass study in 2001, a three-week exercise. I didn't stay there for the entire time; I would come back and forth on weekends. So, I traveled there three times, and Bruce was there. This was before 9/11, so you could put liquids in your suitcase, so I took the wine bottles to Snowmass in three trips, and I had the whole case of wine there, without the wooden case. Bruce and I were going to arrange a little ceremony within the context of this CP physics working group. We'd explain the bet, I would give him the case of wine, we would open the wine and drink it. Then, Bruce's mother got sick, and he left abruptly, so we couldn't have the ceremony. I couldn't do it without him. I had to get the bottles of wine back home, so I gave three bottles to one person, and three bottles to another, and we ferried it all back to Pasadena.
We then tried several times to arrange another ceremony, but we never really found a way. When the accelerator was turned off and BABAR data-taking was done, we had a celebratory meeting—this was in 2009. I asked Bruce to come to the symposium, but he couldn't come. He said, "Why don't you use the wine and save me a bottle?" I thought that was very generous of him, so we did that. We opened the bottles, and it was somewhat haphazard in that some of the wine was not very good because the corks didn't survive all that time; they had never been re-corked. Some of them were good, and the good ones were pretty good, but it was like Russian roulette in opening the bottle and not knowing whether it would be good or not. A couple of years later, Bruce became very ill with cancer, and it became clear he wasn't going to live very long. They decided to have a symposium in his honor, not a memorial symposium, but a symposium while he was still alive, which was a very interesting idea, I thought, at the University of Chicago. So, I took the remaining bottle, and I made a little plaque out of Bob Seimann's note reminding us of the bet and put on it a label from the Château Lynch-Bages and framed it. I went to the symposium, and at the dinner I gave Bruce the bottle of wine and the plaque, which he was quite touched by. I told this whole story to the assembled group. Unfortunately, about two days later, he died. He was really very ill at the time. His wife told me they did open the bottle the next day and did drink the wine. So, that was the denouement of this long, long story. We had a lot of fun not only with Babar, but with our bet.
OK, back to the physics. We made our first definitive CP measurement in 2001 and were ready to have it published in Phys Rev Letters. There was an interesting question because we knew through channels that our KEK collaborators knew of our result. That kind of clandestine stuff happens. If you remember, I told you about that business where we had this measurement of h+- back with Jack Steinberger in the '70s. But the etiquette which we followed back then, and we didn't know if they were going to follow or not, is that if you knew something and you publish, you're supposed to acknowledge it. So, we had a little bit of a set-to involving the editors of Phys Rev Letters to insist that there be, in their paper, which was submitted essentially the same time as ours, an acknowledgment that they knew about our result. After a bit of back and forth, there is in fact a sentence to that effect in their paper—and the papers are published back-to-back in Phys Rev Letters. Not that it matters, it's just that it follows the long-standing etiquette of the field. So, we both then had established that the value of the amount of CP violation in the B meson system, which we had worked so many years to establish, was in agreement with the prediction of the Standard Model. In other words, we were not going to understand with this measurement the baryon asymmetry of the universe, which remains unexplained. But our explanation, this potential explanation was not going to pan out. Nonetheless, it was an important experimental result, and an important theoretical result. It was determined by the powers that be that half the Nobel Prize in 2008 would go to Kobayashi and Maskawa. The other half went to Nambu from University of Chicago, who had done other things involving symmetries. Nambu got half the prize, and then Kobayashi and Maskawa scared the other half, so we each got a quarter. For three people, it's sometimes a third, a third, a third. In the case of the Nobel Prize for gravitational waves, it was again, a half, a quarter, a quarter. They're sending messages when they do things like that. So, Kobayashi and Maskawa were nice enough to invite Jonathan and myself from BABAR and PEP-II, and our opposite numbers from KEK and the Belle experiment, to come to the Nobel Prize ceremony, which was a really interesting experience. We went on to publish very close to 600 papers in refereed journals with the BABAR data.
A lot of papers for one experiment.
We're at it. We're on track to publish perhaps ten more papers, several of them done at Caltech.
Really? There're more papers to write?
Well, the data has still never been superseded. Our dataset from all of BABAR, from 1999 through 2008, and the KEK dataset, which was a little bigger because they kept running a little longer, but they're very comparable in statistics, and our detector was a little bit more efficient than theirs for certain things, they're comparable datasets. So, we just keep analyzing the data because there's nothing better. When we come to B factories, if we do—do you want to stop at 12:30?
No, no. I'm good.
Alright. I thought you said you had a cutoff, or something.
No, not today.
I have a class at 2:00. I don't want to go until then, but we can go another half hour. Maybe that will be sufficient.
Okay. So, even though our accelerator came up to speed quite a bit faster than the KEK accelerator, eventually they wound up getting a somewhat higher instantaneous intensity luminosity than we did, and because they ran longer also, they wound up collecting a bit more data than we did. But as I said, we were capable of doing comparable physics. Nobody can do better in these areas yet. What happened at SLAC was quite unfortunate. Two things happened which were unfortunate. In 2003 or 2004, there was an electrical accident at SLAC in a power supply for the linear accelerator. It had nothing to do with PEP-II; it had nothing to do with BABAR, but there was an electrician who was not following protocols and got very badly burned. DOE decided to shut down the whole lab complex to retrain everybody to a higher level of safety awareness. They kept the entire accelerator complex closed for about a year. They could perfectly well have kept everything running while they did their retraining, but that wasn't the approach they took; they were going to teach SLAC a cultural lesson. In any case, that cost us a great deal of data. We were doing great when we got back on the air, and there was a budget crisis in 2009 for the DOE, right in midyear. There were cost overruns in building certain other experiments, and their solution to the problem was to turn off PEP-II and BABAR. Part of that was the pressure for operating money for the LHC. The American portion of the operating money had to be ponied up because LHC was coming on. So, the danger of closing SLAC for high energy physics which we had evaded in the early '90s finally came and got us. We had a few months grace, so we reconfigured the program and we took some data on some of the other B meson resonances below the ( (4S), which had been unexplored territory. We've actually been able to do quite a lot of good science with that data over the years, so we managed to retrieve something. We could have run for years and could have done even more, but science politics, not the science, eventually got us. We have kept on doing analyses. DOE has not been happy about that. Other countries have varied in this. The U.K. basically shut off work on BABAR quite some time ago. Other countries have been a little bit more sensible. DOE let us do it for a while, and then quite specifically said, "Don't do it anymore." We're still doing it, not on DOE funds. They can't tell us what to do after 6 o'clock. So, we still do it, and we'll do it for a little while more. Maybe another year or two. We'll come to the SuperB factory story, I guess, pretty soon. That will be the end of the BABAR story when that comes up.
Dave, did you feel, as SLAC was increasingly getting involved in astrophysics and cosmology that BABAR was a bit anachronistic in the later years?
The trend toward astrophysics was clear. It was done because one, it was intellectually interesting, and two, because they knew that SLAC was going to be stopped for high energy physics at some point. The exact plan wasn't clear, but DOE doesn't surprise the laboratories; there are long-range plans. They knew this was going to happen. Going into experimental astrophysics was part of the plan. That had started with the experiment which originally was called GLAST and is now called Fermi. NASA always wants to have the last word in naming things, so the LSST telescope is now going to be the Vera Rubin Telescope, etc. Typically, the physicists call it something while they're conceiving of it and building it, and then NASA gets the last word on the name in the end. When the shift started, some people on the Scientific Policy Committee, which is composed of outsiders, resisted SLAC turning in that direction. But the lab knew for its own self-preservation, as well as the intellectual interest of the subject, that it was a direction they were going to go in. It's been quite successful. It has a Kavli Institute there, which was run by Roger Blandford who was at Caltech for a very long time before going to Stanford to do that. So, the timing may have been something of a surprise. The abruptness at the end because of the financial crisis was a shock, but the direction was clear.
Dave, let's move on to the SuperB Factory.
In 2001 we had a meeting in Ise, Japan, in western Japan. It was a meeting from a series which was called BCP, which was set up by the Japanese B Factory folks. We had a similar series of meetings we had been running since the late '80s, which was called Heavy Flavor Physics, the HF series, which had become largely about B physics. They were running one series; we were running another. At the end of the Ise meeting, we had a meeting of people there from the BCP series and people from my series, and I brokered putting the two series together. Rather than competing, why don't we all just do the same thing together? We'll give it a different name, which we called Flavor Physics and CP Violation, FPCP. We decided we would alternate it between Europe, North America, and Asia. Every year it moves around, and it's still going. In fact, next week, there's going to be an edition of it in China, which will be virtual, largely. At that Ise meeting, BABAR showed our first CP results. We were starting to have other results on the B meson lifetime, and several other things. The Japanese collaboration had its results; we had ours. People from Cornell still had things to say. Other people did. One of the sessions one tends to have at these meetings concerns the future. What are you going to do in the future? So, I listened to what people were saying, and typically they were talking about upgrading this a little bit and making the intensity of the accelerator a little bit more. I just looked at it and said, "Well, that's not enough." It's not ambitious enough. If you do a factor of two or three better, you know, statistics goes like the square root of the number of events you have. That's not going to be enough for a funding agency to give you $100 million to do it better; you need to be more ambitious. It turned out that at this meeting, there was another person there from SLAC named John Seeman, who had been the head accelerator person for PEP-II. He was there to give a report at this meeting about the status of PEP-II. I asked John what if we just said we're going to use as many components as we can, but we're going to really rip it up and start over. How well could you do? Could you do 1036? I should give you some numbers. The luminosity goal of the PEP-II accelerator was 3 x 1033 in unites of centimeter minus two seconds minus one. We got that within about a year, and we had plans to get up to, over time, 1034, three times more. We had built that in as kind of a safety factor that we could do with small investments and experience. In fact, we reached that eventually. The Japanese had the same kind of plan, and they were behind us. They caught up to us, and eventually they surpassed the 1034 by a bit, but that was years into the future. So, 1034 was the standard that we were aiming to get to in a few years.
One of the theorists that I mentioned before, Tony Sanda, who had written one of the important original papers, while in the U.S. at Rockefeller University, but had then gone back to Japan, to Nagoya, gave a talk at this meeting, and said that you need more data to do more things and you really need to get to 1034. But he made a typo on his slide, and instead of writing 1034, he wrote 1043. In other words, instead of three times more, seven orders of magnitude more, which was crazy. We all had a good laugh about that, and people often in the future would mention 1043. So, when I went to John Seeman to talk about this, I said, "I won't ask you to do 1043, but how about 1036 instead of 1034?" In other words, 50-100 times more than we're currently capable of. "Could you do this?" He was sitting in the back of this, listening to all the physics, but he was not a particle physicist. He was an accelerator physicist, so he didn't really need to pay full attention. He started thinking about it and making back-of-the-envelope calculations, scaling things. And he said, "I think you might be able to do 1036."
So, when we got back home, we got serious about this. Let's get a couple people together and see what it would take to do 1036, what it would take for a detector that's capable of doing the physics at that much higher intensity, which would mean you'd have to change a lot of things in the experimental apparatus, too. We set to work on this in spare time, basically, because we were all busy with BABAR data and making the accelerator run better. But we started, and eventually it started snowballing. We put people together and I did my thing of running workshops, and we started making progress. The technique was somewhat brute force. The idea was to do it the same way, in the same tunnels, but have much higher current, which would mean it would cost a great deal more to operate, and it would be really hard to provide the cooling. But we thought we could do it. So, we started pushing this, but weren't making a lot of headway. The Japanese started doing the same sort of thing, I think, not at the level of intensity that we were, but they started thinking about it, too. SLAC was not terribly receptive. This time, for different reasons, because they knew the handwriting was on the wall, and SLAC was not going to be in business to do high energy physics in the teens and twenties. They were going to last as long as they lasted, but they were not going to get more funding for a big high energy physics installation. So, SLAC wasn't terribly supportive, but they let us use some of the people to work on this. We wrote a good design report employing the brute force approach. The next interesting thing that happened was one of our Italian collaborators, Marcello Giorgi—who had been a prominent BABAR person, and in the later years had become one of the spokesmen, started talking about this among the Italian accelerator community—and Pantaleo Raimondi—who actually had been at SLAC for a long time, but at that point was back in the Frascati Laboratory outside of Rome—came up with an interesting idea.
A lot of work had been going on in the community on linear colliders, and it still is going on. SLAC worked on linear colliders for many years. There's a candidate site in Japan that they're trying to get support for. That would be a worldwide accelerator. It may happen; it may not. I don't know. But a lot of people had worked on linear colliders. The secret to linear colliders is you need very, very tiny beams with very small cross sections to get the needed luminosity. Instead of big currents, you have small currents, but you have very small beams. That was the original idea of the linear collider B factories that we talked about in the late '80s. That turned out to be an idea that wouldn't have worked. If you remember, I told you that the small ring on big ring turned out to be a dead end. The linear colliders for B factories turned out to be a dead end, too, because those small beams made a very disruptive collision and reduced the effective cross section. B meson production was reduced, so that wouldn't have worked either. But remembering that, Raimondi’s concept was to use techniques that had been developed to make very small beams and put them into circular accelerators. That idea had been used for synchrotron radiation light sources. The accelerator at SPEAR that I had worked on became a synchrotron light source, making the beams very small, so you had a very high brightness X-ray source. Other purpose-built accelerators of that kind were all over the world by then. So, people knew how to do that. Raimondi's idea was to build two small storage rings that used very, very small beams to make high luminosity without terribly high currents. In fact, the optimal way to do that was with rings that were much smaller diameter than the rings at SLAC or the rings at KEK. That seemed like a really interesting idea. We weren't getting very far with the U.S. It was a new idea, so we all made a turn and said, well, let's work on that idea and the detector that would be suitable for that regime, which in fact would be easier to build than the one for the brute force accelerator, and let's see how far we can go with it.
So, we worked on it. This was the concept that we called SuperB. That was Marcello's idea, I think. It was a fine name and we liked it. It has a problem, however. When you Google it, you get the word superb. But other than that, it was a perfectly good name. We worked hard on SuperB. We got ourselves organized; we got involved with the Italian funding agency INFN, which at the time was run by Roberto Petronzio. He was very much on board about this. The idea was we would do this at the Frascati Laboratory, the INFN laboratory at Frascati, which had a small accelerator that ran at 1 GeV, producing pairs of kaons. We would do it there. There was a question of whether you could do it on the site or do it on a neighboring site that belonged to the second Rome university, which was very close to the laboratory. We went back and forth on that, making two designs. The staff of the laboratory, interestingly enough, was not terribly supportive of doing this, for their own reasons. And the director wasn't terribly supportive. But the head of the funding agency, Petronzio, was. So, they did a lot of the requisite Italian science funding policy work, and in 2009, in the wake of the 2008 downturn, there was a stimulus funding exercise in Italy, as there was in the U.S. Included in that stimulus funding was support for SuperB, to the tune of €250 million. That wasn't enough to do the job. We knew that and they knew that, but the idea was to get started and then come back when we had a full cost estimate and talk about the rest.
So, we set to work on doing that, forming a real experimental collaboration, designing the detector, optimizing the accelerator, improving it and improving it. Raimondi came up with good ideas, including something that technically was very innovative, called a crab waist, which was a remarkable improvement in accelerator dynamics, and allowed us really to really be quite sure we could do this without having very high current beams. We were all set to go, we thought. We started trying to form collaborations; we tried to get money from the U.S. for what was now an Italian project, but we would get what we could for support of the U.S. participation in it. In the end, we did not get that. As we made progress over a couple years, we then had a better cost estimate which was more like €500 million. So, the funding agency went back to the government for the rest of the money. And at that point, the government had changed, and they said, "Well, you can't have any more money. You can have the €250 million if you can do it for that, but you can't have a penny more." So, we went back and seriously looked at that. What could you do? What corners could you cut? But we just couldn't do it. You just couldn't squeeze things by a factor of two. So, we adopted a fallback position. We said, "Well, with €250 million, using the same techniques, we can build a really good tau charm factory." A lot of us weren't that thrilled because we didn't think that physics was as interesting as what we wanted to do, but we gave it a shot and then we decided that wasn't really viable either. So, after a couple years of trying, we just gave up.
So, we basically disbanded the collaboration and went on to other things. The Japanese adopted some of these ideas. They were stuck with the big ring. We had optimized the size of the ring at something much smaller. The PEP ring was about 2.2 km in circumference. The Japanese one is about 3.1, much larger than optimal. The design we had optimized for SuperB was something about 0.9 km in diameter. That has a whole bunch of advantages, including making it cheaper because it's smaller, but also a lot of advantages in beam dynamics. In Japan, getting civil construction is much more difficult because of the way they're organized than building apparatus that fits into an existing piece of civil construction. So, they decided they had to do this within this big tunnel. And using some of Raimondi's ideas, they designed an accelerator to do that. They were working simultaneously with us. We're very competitive but we're also very collegial, so we exchanged information. We had joint workshops. Some of them we made in Hawaii because they were halfway between, and everybody likes going to Hawaii. We had workshops in the U.S. on the west coast, we had them in Italy, we had them in Japan. So, we were very much in contact.
We had a lot of qualms about their design, largely because of some of the inevitable consequences of trying to do this technique in a very large ring. They didn't agree with us, but we made our concerns known and they took some of it into consideration. When SuperB failed, our group basically scattered to the four winds. Frank and I, because of these qualms, decided that we would—typically, when these things happen, the losers are free under certain circumstances, assuming they haven't been fighting, to join the winners. So, some people from the BABAR collaboration and the SuperB collaboration actually went to work on the Japanese experiment, which is called SuperKEKB and Belle II. They are running now, but we didn't join that. Frank and I decided to join an experiment called Mu2E at Fermilab, which I can talk about briefly in a little bit. Others joined an experiment at CERN, and some number of groups, including a few U.S. groups, joined the Japanese experiment. DOE was willing to fund a small number of groups to do a little bit there. They've been running at various levels for a couple of years now. They have now reached the same luminosity, the same intensity, that they had when they shut off KEKB, back in 2011, or something like that. Their goal was to get to not quite 1036, but something like 5 x1035. They're not there yet; they have a long way to go. Their technique, in order to get to higher luminosities, they have to shut down the machine and add RF stations, which are quite expensive things and require you to rebuild a lot. So, their plan to get to higher luminosity requires run, stop and build, run, stop and build, which makes it very hard to accumulate a lot of data in a short period of time. I have always regarded that as something of a flawed plan, which is one reason I wasn't enthusiastic. The other thing that's hurting them is they have a lot of backgrounds, particularly one background called Touschek scattering background. Our work on SuperB was largely concerned with working on controlling Touschek backgrounds. That was one of the main things that we kept saying in your design in the big ring is going to be a big problem. And in fact, it's a big problem. It's those kinds of backgrounds which are making it difficult for them to get up to higher luminosities. The accelerator is capable of doing so, but only while producing so much background that the experiment can't run. So, it's been a real problem for them. It doesn't mean they can't solve the problem. I hope they will solve the problem. But they haven't really yet. They are working diligently to bring things up to speed to get to accumulating this much larger dataset, but they've made rather slow progress at it. So, the dataset they have accumulated now is about the equivalent of what we got in the first year of running PEP-II and BABAR in 1999 and 2000. That doesn't do anything new, since both their older experiment and BABAR have 15-20 times more data than that now in the can. The assumption is that over time they will get to where they want to go, but I decided I didn't want to go on that journey, and that it would be best to turn to something different.
That something is Mu2e. As I've told you, the name of the game is to find physics beyond the Standard Model. We have measured CP violation nine ways from Sunday in the B system, and we have found no evidence of physics beyond the Standard Model. There are modes of B decay that are more sensitive to physics beyond the Standard Model, but they require a great deal more luminosity to study; they require a SuperB factory level of luminosity. So, that's what SuperB and SuperKEKB are about. These are rather small effects; the effects that they produce are governed by the scale of the new physics. At the time we were working on all this stuff, from 2000 to 2011 or so, we didn't know anything about the scale of new physics. Now, from the LHC results, we do. We don't know in detail anything about the new physics, but we do know if it exists, that it has a mass scale which is higher than the SUSY proponents, for example, had hoped. Typically, the mass limits on various new particles which could be involved is up above a TeV, or so. If you take a TeV scale, and ask how big could these effects be in B physics decays? The answer is, they're really small. So, at least in my book, the situation has changed. Being able to make those kinds of CP violation measurements at a Super B factory, knowing what we know now about the mass scale of new physics, is now very problematic. Assuming they get the accelerator working, and they take all the promised data, and they make these measurements of a very small asymmetry, it's very unlikely to be affected by new physics at a scale that can be measured. I could be wrong about that; I hope I'm wrong about that, but my best physics judgment is that CP violation effects due to new physics in the B sector are out of the experimental reach of Super B factories. It was that kind of consideration which led me to think that maybe I don't want to do that anymore, that I would like to find a sharper tool to look for new physics. Now, I should put a caveat—there are, in the BABAR data, in data from the LHCb experiment at CERN, some indications of perhaps violations of the standard model in B physics. Not in CP violation in B physics, which is what I was talking about, but in other areas. In results involving lepton universality. In other words, the decay of B meson to tau, mu, or electron in a final state, along with other things, should obey the Standard Model rules called lepton universality. We should know exactly what those ratios should be. In some of their measurements, that's not the case. But the effects are small. They're 2 sigma kind of effects; that is, they're not fully established. But they persist as they get better and better data, but they're still never really statistically significant. And the BABAR and Belle data tends to be more in agreement with the Standard Model, but not very good either. So, whether, in fact, that is evidence of new physics in the non-CP area, remains to be seen. Jury's still out. But again, I thought it would be important to look for a sharper tool.
Dave, what does it mean, a sharper tool? What specifically are you thinking about?
An experiment that has sensitivity to effects due to even higher mass scales, like Mu2e. Mu2e is basically a single purpose experiment. It's aimed at finding charged lepton flavor violation, not at a wide spectrum of areas, as with BABAR. When you look at how new physics could manifest itself in this area, the scales to which it's sensitive are much higher than those that B physics and charm physics and the things we've been doing are sensitive to. It can find effects if the scale of new physics is 1000 TeV. So, you have this much greater reach in the scale of new physics if you look for this non-standard model effect in muon decays. The reason for that is not magic. It has to do with the fact that it's easy to make a muon, which are copiously produced in pion decay. It's very easy to make lots of pions if you have intense proton accelerators. You can therefore study a much larger sample. You can accumulate so many muons that you can look at very, very, very tiny effects. The effects we're looking for are a part in 1016. In fact, in order to get to this 1016 level, you need to look at least at 1016 muons to find a single example. We stop 1010 muons in our experiment every second. A year has p x107 seconds. So, if a single experiment were to run for a year, it can have more than 1017 muons to measure. Accounting for efficiency effects, it can have a sensitivity in this range, 10-16 to 10-17. The magic is due to the fact that you can look at so many more muons. We make and look at more muons in a day than SuperB can study in a decade, for example. It's as simple as that. So, Frank and I decided to go in that direction and join a just forming Fermilab collaboration.
There are different ways of looking at these effects in muon physics. We have chosen the one that's called muon to electron conversion. That is, you bring a muon into the field of a nucleus, and it turns into an electron. Now, muons turn into electrons all the time; they decay to an electron and two neutrinos. I talked a little bit about that early on. That's allowed by the Standard Model, but for a muon to become an electron with no associated neutrinos violates flavor lepton conservation. So, that's the experiment. People have been doing these searches for many years, even back in the days of cosmic rays, when you only found muons in cosmic rays. The current experiments have a level of sensitivity for muon to electron conversion of about 10-12. In order to do better, some Russians had an idea back in the mid '80s and designed an experiment that was supposed to run at a Russian accelerator at the time that was never built. The idea was resurrected in the United States and the host for it was to be Brookhaven National Lab, which has a 30 GeV proton accelerator, which goes back to the '60s, but is still used as an injector to the heavy ion collider. The idea then was to build this in the style of this Russian experiment, a very sensitive experiment for Μ to e conversion, and it became part of a package of rare decay experiments which was called RSVP which was to happen at Brookhaven. Eventually RSVP became too expensive for something to do at Brookhaven, and it was cut off. The whole program was just stopped. Again, it was part of this business of where the operating funds for the LHC are going to come from. And the other place they were going to come from was residual physics at Brookhaven.
So, a new collaboration formed, built around exactly the same idea, but to be done at Fermilab. That's what became the Mu2e collaboration. So, it was forming with a nucleus of people who had been part of the RSVP work, in the late aughts and the early teens of this century. Frank and I started looking at this, and we decided it was interesting physics, number one, and number two, something we could contribute expertise to. I don't like to just walk into an experiment and say, "Here I am. Give me some data to analyze." I like to build the things I then work on, and then do the physics. That used to be the standard method of operation. It no longer is for a lot of people, but it is for me. So, we wanted to see what we could contribute to this experiment in early phases, and we have a lot of expertise in calorimetry in general. Not only liquid argon calorimetry, but also measurements using scintillating crystal. BABAR's calorimeter was a set of scintillating crystals which were made using thallium-doped cesium iodide. A good deal of the early development work for the BABAR calorimeter was done at Caltech. We actually didn't then participate in the building of that in BABAR, because I had my hands full as the spokesperson, directing the project. But we did a lot of the early development work. We also needed a calorimeter like that for SuperB, but it needed to have much higher speed.
So, we did some work on that for SuperB using un-doped cesium iodide, and other crystals such as barium fluoride. So, we were quite expert at this. We had built a prototype that we ran at a CERN test beam for SuperB. When we joined Mu2e, we started immediately looking at the calorimeter. There's a bunch of different apparatus that make up Mu2e. It is made up of three superconducting solenoids. Very complicated things which are all being built now, and some delays are being caused by production difficulties with that, which is another story. But there is a tracker, and a calorimeter that tells you that the outgoing particle we're looking for was indeed an electron. It looks exactly like an electron because of what it does in the calorimeter. The design for the calorimeter was a survival from when the original RSVP detector was being sketched out, and people were starting to do tests of different kinds of crystals, and readouts for the crystals. So, we started looking at how we would fit our knowledge of crystal calorimetry into this.
I first started looking at the geometry: how do you configure the crystals, and I decided that the design that was on the table was not optimal. I came up with a different design, arranging the crystals into two annuli, which has a bunch of advantages over the original. It has higher geometric detection efficiency, and, importantly, it is symmetric to negatively and positively charged electrons. There's a class of experiments that you'd like to do that detects positively charged electrons, and the old design couldn't be used for that. The design I put forward could be used for both. So, we went through a process to make a design change. You have to prove the superiority of the new geometry to a bunch of skeptical experts, which we did. That was one of our major initial contributions to the experiment, along with our expertise on crystals. Our collaborators on the calorimeter are groups from Italy, but not the same groups who had been working on SuperB. A bunch of different groups with similar expertise; we've been working very well together. Most of the money for the crystals and the mechanics is in fact coming from Italy.
So, we share the development, but a lot of the actual building is being done by the Italians, and we're doing a portion of that. It's been working, and we're almost ready in the next few months to actually start doing the assembly. We have all the crystals. They've done all the testing and setup of half the crystals. We've done the testing and setup of the other half in Pasadena. All the crystals are now at Fermilab, and as soon as Italians can get visas to return to Fermilab, because of COVID, we'll send people and they'll send people, and we'll start the actual assembly of this device. There have been some delays, not terribly serious, due to these superconducting solenoids. There were, as I said, three types of solenoids. The middle set were being built in Italy, a group of toroids, by the same company that built the BABAR solenoid. That's an interesting story, which I should go back and spend a few minutes on. We were very serious about the BABAR schedule. Nothing could stand in the way. In order to meet the schedule of the solenoid being made in Italy, we needed to have it installed at SLAC on a certain date. There were some delays in Italy in the construction. The original plan was to ship it on a boat, which would have missed the schedule by a couple of months. So, we had an idea. The long-term Associate Director of SLAC was a theorist named Sid Drell, who was active in the founding of SLAC. He had done a lot of classified defense work. One of the things he did was work on—in fact, I think he was the progenitor of this idea—was spy satellites that took photographs and then released film canisters that were caught in a net by airplanes, and then provided state of the art surveillance via satellite. Because of this project he had lots of contacts in the Air Force. It was a time when there was NATO bombing in the former Yugoslavia, so there were lots of planes going back and forth. In fact, they were going to an airbase in Genoa, and the factory that made the solenoid was in Genoa. There were lots of C130s, the big cargo planes that can swallow tanks. So, Sid used his contacts in the Air Force to get us authorization. The C130s were going to the Genoa NATO base, and then coming back empty. So, the idea was—and it needed of course the concordance of the Air Force, which Sid procured for us—that they would, on a return flight, take the superconducting solenoid, put it in a C130, and bring it to Moffett Field, which is just south of SLAC. So, they did this just after Thanksgiving in 1998. We met the plane at Moffett Field, and they opened up the big mouth of the C130, put it on a truck, and we kept our schedule because of Sid Drell's contacts in the Air Force.
Back to the Mu2e magnets. The same company has built the toroidal magnets for Mu2e, and they've done them pretty much on time. They're all at Fermilab undergoing testing and acceptance. The contract for other two magnets, the production solenoid and the detector solenoid, which are two really big, complicated things, was won by General Atomics, a large US company that, among other things, makes some of these big, deadly drones that are used by the Air Force. They also make normal and superconducting large magnets. They make them for catapults for aircraft carriers. Not the old steam kind, but electromagnetic catapults that require you to collapse a very strong field. They also make superconducting solenoids for ITER, the big fusion project in France. So, with that expertise behind them, they won that contract to make our solenoids. We thought they would do this in San Diego where they make their superconducting solenoids, but they made a business decision to make them at their other factory which is in Tupelo, Mississippi, which makes normal conducting solenoids, and had never made a superconducting solenoid. We protested that strenuously, but we can't tell them how to do their business. Moving this production to Tupelo, Mississippi, was not a wise decision. The learning curve has been very long. They're now almost two years behind, and they've finished nothing yet. We're in negotiations now to see if we can incentivize them a little bit. It's a fixed-price contract, which they're not used to doing, having usually dealt with the Defense Department. We're working all this out, but it's causing delay. We had some float, so it isn't that we're two years delayed. But they've basically used up all our float due to their business decision. The rest of Mu2e is being built. The tracker part of it is being built now at Fermilab and in Minnesota. We're up to the assembly of the calorimeter, and there are a bunch of other things that have to be built. But we're about perhaps two years away from starting to take test beams to commission the experiment. It will take a while to understand things. It's all about having good efficiency and understanding subtle backgrounds, which can take a while. But we're paying a great deal of attention to that, so I think that's going to work out.
Dave, how did you see LDMX fitting within the larger context of the search for dark matter?
That’s another thread. One thing we did in BABAR, which I haven’t talked about, was, although it was nowhere near the intention of the design of the experiment, some people at BABAR, one of them Yury Kolomensky from Berkeley, early on, and then later Bertrand Echenard from Caltech, found ways to use the BABAR data to investigate dark matter particles in a very different way than is typically done where you have germanium detectors or liquid xenon, and you look for dark matter in the solar wind, if you like, in the wind of the Earth moving through the dark matter, to collide with something that you can identify as a dark matter collision. This is a different idea, and here, you talk about typically a different kind of dark matter, what's called hidden sector dark matter. Not WIMP dark matter. So, it typically has a lower mass scale. WIMP dark matter has now got pretty stringent limits, and it doesn't look like it's a viable idea. So, the idea has turned to dark matter of a somewhat lower mass. It turned out that you can use the BABAR data to investigate decay, e+ e- interactions, that produce what are called dark matter force carriers. That is, gauge particles that connect normal matter to dark matter. There are many different kinds. Dark photons, dark axions, dark Higgs, dark scalars, dark this and that, and they each have different signatures. It turned out that in BABAR you can use our data to set, typically, the world's best limits on many, many of these dark matter candidates. So, almost all of that work has been done by Bertrand Echenard at Caltech.
So, the question is, how do you take a step further in that? You can of course do that with more data. That will happen at SuperKEKB eventually. They'll have, in a decade or so, 50 times more data, and they'll be able to do even better limits and maybe find these dark matter force carriers. But there's an even better way to do it, and that's to do what's called electron scattering from a fixed target. And that's what LDMX is. People are interested in doing—in these lower mass ranges, it turns out you can get very sensitive if you use an accelerator to produce the dark matter, somewhat in the way that we do at e+ e-. But what I mean is different from statically colliding the dark matter particles with xenon, argon, or germanium, things like that. So, we looked at that, and again, Bertrand has been the motivator of this, and found this LDMX experiment, which is a follow-on to an experiment that's happening now at Jefferson Lab called HPS, which is a different technique but it's kind of related. It's currently running. Some of the people doing that, in fact, were thinking about this LDMX, a different way of doing this. So, we know all those people from years ago, working at SLAC. So, we started thinking this looks very interesting. It looks like it's very sensitive. It's a small experiment. It's not a several hundred-million-dollar experiment. It's a sort of $10 million scale thing. It's small. It can be built by a relatively small collaboration. It can be built with very little RND. The tack we've taken is to use apparatus techniques that have been developed for other experiments. Some of the people are involved in CMS/LHC experiments. Some of the stuff developed for that can be taken and used for LDMX. Our idea was to take a part of Mu2E, which is called the cosmic ray veto, and repurpose that technique for the hadron calorimeter that's built in LDMX. So, we could leverage expertise, get involved in a small experiment that can be done without a ten-year investment of people and time, and do something really very interesting, sort of close by home. So, we decided that would be interesting, and we've gotten into it. We got a small amount of money from DOE to do the development work and some of the engineering. In fact, next week, we're making a presentation to DOE to see—a progress report on how we've spent our money. That's part of the campaign to try to get them to really fund the experiment. There are six candidates for what they called the Dark Matter New Initiative. Not all of them will be approved, and we're hoping that LDMX will be one of them. We won't know that next week, but it will be part of the journey toward approval which we hope will happen in a year. I'm reasonably confident that we'll be one of the winners, because one, it's a very good idea, and two, we are doing very well on development. So, I've become what's called the technical coordinator of the experiment. I'm not the spokesman, but I'm in charge of seeing to it that the jigsaw puzzle fits together, if you like. That the tracker and this part and this other part are commensurate. We set requirements that techniques used meet the requirements, that this guy doesn't step on that guy's toe. All of those sort of detail things, which I've been doing for longer than anybody can remember. So, that's my role in this thing. So, I'll be making a presentation next week.
Dave, the question is too irresistible not to ask. If you had to choose, are you more bullish on new physics beyond the Standard Model in particle physics, or finally figuring out what the dark matter is?
They're different. They're two very different—I mean, you could find dark matter that is a material that interacts primarily gravitationally, but actually is particles that have connection to Standard Model particles, and that says nothing whatsoever about beyond the Standard Model with supersymmetry or something else. They're just two different compartments, if you like. They could be related; people thought that these WIMPs could be the lightest supersymmetric particle. That's now unlikely. So, they could be related, but they are not necessarily related.
Dave, what was it like winning the Panofsky Prize?
That was fun. That happened in 2016. It was enjoyable. We went to an APS meeting in Salt Lake City and made presentations. There were four of us, the two from Belle and Jonathan and I, so we divided up what we talked about. We tried to be coherent, and three of the four of us actually were. Our friend, Fumihiko Takasaki from Belle, gave a talk about something else entirely. I never figured out why. One of the interesting stories I'll tell you about—this is funny, and I had nothing to do with it, but I learned about it. In the wake of doing successful experimental things, people are nominated for prizes and whatever. So, John Seeman, who had a lot to do with the accelerator, got the Wilson Prize for accelerators. There was no prize for B factory experiments, even though it was recognized as important physics. Several years after, and I don't remember—2004 or 2005 or so—a Panofsky Prize was given to Pier Oddone for the idea of the asymmetric B factory. It was very well-deserved, and everybody was very happy about it. But the question was why? That was just a piece of what had been done in order to measure CP violation in B physics. So, what happened? I had no idea. I actually found out one day a couple years later, in having a conversation with Ian Shipsey, the chairman of the Panofsky Prize committee at that time. I have been chair of that committee several times over the years. I didn't give myself the prize, of course. Ian told me that there was a debate about this because they thought there should be a prize for CP violation in B physics, the experimental aspect of it, but the problem was that the obvious people to award it to were the four people I've been talking about. Jonathan and I from the BABAR/PEP-II side, and Steve Olson and Takasaki from the Japanese side. But you can't give the prize to more than three. So, they didn't know what to do. They had a long discussion and decided to compromise by rewarding the idea of the asymmetric B factory and giving it to Pier Oddone. That was their Solomonic decision. So that was the end of that.
Then, many years later, in around 2014, I was at one of our FPCP meetings in Brazil. At a coffee break, Ikaros Bigi who was the theorist who wrote the important paper with Tony Sanda came up to me; he and I had known each other forever, back to the charm days with Mark III, and he's a rather direct guy. He said, "How come you don't have a Panofsky Prize?" So, I told him this story, and he said, "That's not right." Okay. So, he drank his coffee, and I drank mine, and that was the end of it. Then, absolutely by happenstance, about six months later, I was in Novosibirsk at a meeting, and at the dinner afterwards, where there’s more vodka than there is food, a couple of my old friends from Novosibirsk came up to me and said, "How come you don't have a Panofsky Prize?" So, I told them the story as well, and they said, "That's not right." So, they went and had another drink, and I went and had another drink, and that was the end of it. But apparently, what happened then is one of them, Valery Telnov, who I've known forever, as he was one of the first visitors to SLAC when we were building Mark II, and was responsible, in fact, for the first physics paper that Mark II produced at SPEAR, did something. His son was a graduate student on BABAR. Valery got together with his son, and they, together with others, I don't know whom, wrote another nomination for the Panofsky Prize. I don't know who they nominated. I presume the four people. The wheels then turned, and we got the Panofsky Prize. I only found this out afterwards. I didn't know that was happening at all, but it later came up in a conversation with Alex Telnov, Valery’s son, who told me that he had a lot to do with the writing of the nomination. Then at another meeting, I ran into Bob Swoboda, the chairman of the Panofsky Prize committee that awarded that prize. He said, "Congratulations." I said, "Thank you." And we started talking. I said, "How come this prize was awarded to four people? How did that happen?" And he said, "Well, we had this discussion because there was a nomination, and the same thing came up, that you can't give this to more than three people. So, I called up the APS and I said, 'There's this limitation. Can we do anything about it?' And they said, 'There is no such limitation.'" In other words, this restriction had somehow been adopted, because that is a limitation for Nobel Prizes, and somehow, everybody had just made the unfounded assumption that it applied to the Panofsky Prize. There was in fact no such barrier, so they did it.
Dave, we talked about your current work right at the beginning of our talk last time, so let's end looking to the future. What is most important to you? What are you most curious about in physics? What issues have gnawed at you over the course of your career for which you are optimistic that fundamental discovery remains to be found?
Oh, I'm certain fundamental discoveries remain to be found.
But the question is, what are you most optimistic of being a part of it?
Well, I've taken my best shot. I think, right now, Mu2e is my best shot, and LDMX is a pretty good shot at dark matter. One goes into these things after thinking hard, because they're big commitments. It's not like you do an experiment for a year. We're back in Mel Schwartz's realm. Remember? Would you do the experiment if you weren't going to be around to see the answer? That's where we are. And in fact, that may be the case. I mean, I'm 79—I'm still functioning pretty well, and I'm hoping to be there for at least the beginning of the physics in both these experiments. We'll see how far I get, but that's the goal. You know, you do these things a step at a time. I've done a lot of different areas of physics, but if there's any thread, it's one that really doesn't begin with my thesis physics, which was about muonic X-rays. It's interesting that Mu2e involves a muonic atom with aluminum, but that’s not the central physics. In fact, we're starting to think about the follow-on to Mu2e-II, which will involve other nuclei. And I actually know a lot about that from my thesis days, so I'm actually working on that. I certainly won't be around to do Mu2e-II, but I can still contribute something to the design of that experiment because of that expertise. So, I'm working with a student, in fact, right now to do that. But as I said, the through thing I would trace comes not from my thesis experiment, but from my thesis advisor. That is, Chien-Shiung Wu did the definitive work on understanding the weak interactions. She's known for the parity experiment, but she did much more. I think I mentioned a bit of that in the beginning. She was the single most important person in figuring out the Lorentz structure of the weak interactions, with a series of nuclei, with refining the experimental techniques, with proving the conserved vector current hypothesis. So, that's the thread I have actually followed, working on K meson decays, weak decays at SLAC, the charge asymmetry work, which was Mel and Stan's idea, not mine. And then the form factor stuff, which was something I did. And then, with some byways, like the KLp interactions with the rapid cycling bubble chamber, which was a technical opportunity which was too good to miss. And then going into e+e-, which covered a lot of ground, but the part of it that I've always followed was doing weak interaction studies with e+e-. In fact, most of what we know about charm physics and B physics comes from e+e-. So, there were always lots of physics that we did, and I was involved in, but the thread that goes through is weak interactions. I followed that at SPEAR; I followed that at BES in China; in BABAR those were my primary interests as well, and at SuperB, and the same is true with Mu2e. So, if there's any connection in what I've done and what I've accomplished, it's in that area. And that's all due to Wu.
Once a student of Madam Wu, always a student of Madam Wu.
Yeah, I mean, she was focused, and my approach has always tried to follow—I always say, "What would she do?" When we were designing the electromagnetic calorimeter for BABAR, it's what's called a projective calorimeter. The individual crystals point at the interaction region. People always worry about photons going through the cracks, because there were always cracks. So, we decided we had to design it so that the cracks did not present themselves to the interaction region. This is going to be really wonky. In order to do that, you can make it non-projective, slightly, in either the polar angle, or the azimuthal angle. So, the one thing you would always do is shift the azimuthal angle. We were doing that, but then others wanted to also make it non-projective in the polar angle. I thought about that, and I came to the conclusion that this would make the apparatus non-CP invariant. And the whole experiment was meant to measure CP violation, so what I finally said was, "I'm a student of C.S. Wu. If I were to try to measure a CP violating quantity having purposely built a non-CP symmetric detector, she would spin in her grave. I can't do that." So, we made it symmetric in the polar direction. It's that kind of attention to the detail of an apparatus which I learned from her.
Dave, it's been an absolute pleasure spending all of this time with you. This has been an epic conversation, and I'm so glad you've shared with me, in such great detail, all of these stories, which is amazing because I've heard generally so much about this, but not nearly in this level of detail. So, this is an immediate treasure for the archives. Alright, Dave, thank you so much.