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Interview of Gary Feldman by David Zierler on June 19, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
In this interview, Gary Feldman, the Frank B. Baird, Jr. Research Professor of Science at Harvard University, recounts his childhood in South Bend and his undergraduate experience at the University of Chicago. Feldman describes the opportunities that led to his graduate work at Harvard to work with Frank Pipkin on electro production pion experiments. Feldman discusses his postgraduate research at SLAC where he worked closely with Roy Schwitters in Burt Richter’s group measuring the form factors of baryons and pions. He describes the similarity of experiments connection Richter’s discovery of the Psi and Martin Perl’s discovery of the Tau twenty years later, and he describes the SLAC LDL detector project and the impact of LEP collaboration on SLAC. Feldman explains his decision to join the faculty at Harvard and the status of the CDF experiment at Fermilab at that point. He discusses his contributions to the NOMAD research at CERN looking for the tau neutrino in an electronic bubble chamber, his work on the MINOS experiment at the Soudan mine, and he explains the problem of CP violation in terms of what one can see with neutrinos and anti-neutrinos. Feldman prognosticates on future work to determine evidence for a sterile neutrino, and he offers his perspective on the downfall of the SSC and why Burt Richter’s directorship may have made the difference. At the end of the interview, Feldman points to Japan and China where some of the most interesting high energy physics is happening, and he notes the value that particle physics is contributing to deep learning and artificial neural nets.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is June 19th, 2020. It is my great pleasure to be here with Professor Gary Feldman. Gary, thank you so much for being with me today.
I'm happy to do it.
All right. So to start, would you please tell me your title and institutional affiliation.
Well, my current title is the Frank B. Baird, Jr. Research Professor of Science at Harvard University.
Okay wonderful. And now, let's take it right back to the beginning. Tell me a little bit about your parents. Where are your parents from?
My father was born in Poland, came to the United States as a child, grew up in New York.
What year did he get here?
It would have been shortly after-- I don't remember. I have it, but I don't have it with me. It would be shortly after the First World War. His father had come earlier, and after the war the rest of the family came. He was, I think, the only one of his siblings to go to college. City College of New York. And went to medical school initially in Königsberg. And that would have been in 1933, and he decided that Königsberg in 1933 wasn't a good place to be. So he transferred to Basil, and he did his medical training in Basil, came back to the United States, worked for a while in the Conservation Corps as a physician. Then joined the army, I'm not sure exactly why, again as a physician. And was stationed in Cheyenne, Wyoming. Outside of Cheyenne, Fort Warren. Fort Francis E. Warren.
And he had a broken-down car, and he kept driving into Cheyenne to get parts for it, and the owner of the store was a guy named Charlie Rosenberg, and he introduced my father to who became my mother, who lived in Cheyenne, and they got married.
But where was your mom born?
She was born in Salt Lake, actually. My grandfather came from Russia. Again, as a teenager, the usual reasons, you know, avoiding the draft, and so forth. And was kind of a rough and ready guy. So he came initially to New York and then went to Texas. The story he tells, now he'd tell stories that we can't verify (laughs) was that he got into a fight in Texas with someone and apparently injured them. Decided it was a good time to get out of town, went to the railway station, put all his money on the counter and said, "Where can I go for this?" And the guy said, "Well, you can go to Salt Lake. You can have as many wives as you want there." He said, "That's for me."
So he moved to Salt Lake. Eventually married my biological grandmother, who lived in Salt Lake, and died in the 1918 flu epidemic. And he eventually remarried and moved to Cheyenne. So I was actually born-- What happened, so my parents put in for a transfer to Hawai'i, and they got it. And they apparently were living a very nice social life of the officer corps in Honolulu until Pearl Harbor got bombed. And they were there for that. My mother was pregnant with me.
Did your father assist with medical relief?
Did your father assist with medical relief after the attack?
Well yeah, he was in the army. He spent the rest of...yeah. He was in the army, the attack was actually on the Naval base. But it's only a couple miles from the hospital. And he spent the rest of the war in the South Pacific. So I got evacuated. My mother went back to live with her parents, and I was actually born in Cheyenne, or at least at the army base.
Afterwards, after the war, my father was looking around for a practice, found one in South Bend, Indiana, where his New York license would be valid, and he thought, well, it's a, you know, it's a college town of sorts. Really the Notre Dame doesn't have too much influence on the city. But it was a nice thing to have there. And so I grew up in South Bend, which was a little provincial, but it was very pleasant.
The one thing that, you know, one question might be how did I get into physics, and I was not a, you know, sort of a gadget guy in high school. Some physicists would build, you know, cloud chambers and things like that in their basement. I never did any of that. I did participate in a mathematical contest that the state of Indiana had at the state level.
Gary, did you go to public schools?
Did you go to public schools throughout?
Yes, I went to public schools. I went to South Bend Central, which was...I think I got it slightly wrong. It may have been the year earlier. This famous movie, Hoosiers, I don't know if you know—
Sure, absolutely. Yeah.
I thought that South Bend Central was the team that they beat in the finals. I think actually South Bend Central won the finals the year before. And got a good education there, I think. And what got me a little more interested in physics, actually, was an open house that Notre Dame had. And I went there as a senior, and I said, you know, this is kind of interesting.
I went to undergraduate school at the University of Chicago. I was impressed with their general education scheme, that... Now I'm trying to, I'm blanking. As a senior, I'm blanking on all these names. Hutchens. Yeah, Hutchens, had instituted. And in fact, he was a very good gentleman, patient and classical to general education. And what I decided is, well I'll start in physics, and if I find something more interesting, I'll change.
And I never found anything that interested me more, and I was led to the simplest, most basic physics. In other words, so I wanted to know, you know, what is the most basic things in physics as opposed to other types of physics, atomic or solid state, that or chemistry, that seem more complicated to me. So I went in that direction. After Chicago, I went to Harvard for graduate work.
Well Gary, before we get on to Harvard, I'm curious. When you were thinking about going to Chicago, did you have an awareness of just how world-class the physics program was there and what an amazing history it had, you know, during the war and afterwards?
I don't remember to what extent. I'm sure I did know a lot about it, yeah. I had biology labs in this place where Fermi had built his, under the stands of Stagg Stadium, where Fermi had built his pile. So I knew some of that also. It was, I have to say, Chicago was a very monolithic type. The student body there was fairly monolithic. I found a completely different student body when I actually got to Harvard.
Monolithic in what way?
Well, there's a sort of a chestnut saying, that the University of Chicago is a Baptist institution, where atheistic professors teach Jewish students Catholic theology. It was exactly right. I got to Harvard, I discovered an undergraduate, a much broader set of undergraduate students. Who I liked very much. They were very, very broad. And you had all sort of people.
Gary, what prof--
They were real scholars. The entrepreneurs, the more social class and so forth.
Were there any professors at Chicago that you became close with in the physics program?
Not terribly close. One that, oh god, I'm going to... I took two terms of thermodynamics from... I'm missing on his name.
You can always catch it for the transcript, don't worry about it.
I can always substitute it in. And I got to know, very interesting, I got to know his son also, because he married a woman from South Bend who happened to be the sister of Jonathan Pollard. And in fact, Jonathan's older brother was a contemporary of mine, and we actually roomed together one summer in Chicago. He went on to become a physician, PhD physician. And I don't remember exactly his work in government institutions.
Gary, I'm curious. When you were starting to think about graduate programs, how well-established was your identity in terms of the kind of physics you wanted to work on? Did you gravitate more towards experimentation, or more towards the theory at that point?
I gravitated towards the experiments, and basically, I didn't feel that I had the, you know, I knew people that were going into theory, and I thought these people really understand it better than I do. So I think I can make more of a contribution in experimental work. And even though my original, you know, in high school, I mean mathematics was certainly more interesting to me, the level of theoretical physics was, I thought, you know, something I could do, but I didn't think I could excel at it. So that was the reason.
And I don't remember exactly what led me to Harvard. Part of it was that I knew that they had the Cambridge electron accelerator there. And so then presumably, I could do research, you know, locally. Which turned out to be correct.
So Gary, so this means accelerator physics was sort of on your radar from the beginning for graduate school?
Yes, yes. Yes. And so that was, you know, the choice I made. So at Harvard, I started working with Frank Pipkin on some electro production experiments, and you know, this was very small groups at the time. We had about five or six graduate students. We had no postdocs for a while. Towards the end, we had one or two postdocs, one of which we, the graduate students that's assigned to do the range of corrections, and the other sort of stayed clear of the graduate students. He was helpful in terms of giving advice, but he was looking towards future programs rather than interfering with what we were doing.
So it was also a great education for a graduate student, because we did everything. We built the experiment, we wrote all the software, we did the analysis. We basically ran the thing. And that doesn't happen so much anymore. Graduate students end up getting into these very large collaborations, and they get, you know, into one little slot. Maybe one slot in terms of the instrumentation, and another one in terms of the analysis. But so that worked out well.
So anyway, I did my thesis on electro production of pions. It took a while for two reasons. One was he had this explosion at the CEA the year after I arrived. In fact, it was the summer after I arrived. And that basically took the whole laboratory down for a year to get back on track. The other was that Frank Pipkin had sort of, at one point I came into his office. And he said, "Well, you know, we've got this project I want to do at SLAC. Maybe you'd be interested in that?" So he talked me into it, and we wrote a proposal. We got it approved at SLAC, it never got run. So I ended up again working back at the Cambridge electron accelerator. But it was a nice thesis. It was basically a measurement of the pion size, pion form factor technically. Using electro production and theoretical model.
So I started looking around for a job afterward, a postdoc job, and I got an offer from SLAC. Now, what had happened there was interesting. An older member of Pipkin’s group, well he was friendly with, had gone out to SLAC and was working for Martin Perl there. And he said that Martin was looking for postdocs and having trouble finding people he liked, and so my friend recommended me, and I interviewed out there and I got an offer.
An interesting thing, which I still remember, is that I also, among the other places I interviewed was at Argonne National Lab, and I had spent a couple summers working at Argonne as an undergraduate on two different things. One was an experimental nuclear physics project, and the other was actually a theoretical project, where I was doing some programming. And one of the, I've forgotten his name now, but one of the old timers there that I interviewed with told me, he said, "Well, if you have an offer from SLAC, you should take it. Because they're better than we are."
Better across the board? Or better particularly in what you were interested in?
Well, I mean they do high energy physics. They were better at high energy physics. And that was good advice. And so what happened, I mean I think that a lot of the success I had was the fact that I just ended up in the right place at the right time. Went out to SLAC. I remember my wife and I drove across country in April, 1971, after I finished my thesis. So I started, I mean I said it took a while. I started in 1964, fall of 1964, so you can see that it was not the five-year recommended timescale. That was the only time I've ever driven across country. And it was enjoyable.
I remember one interesting thing. We stopped, of course, at various places to visit people or visit things. And we stopped at South Bend to visit my parents. And my father was a physician. I guess my wife was wondering if she was pregnant or not, and so he did a pregnant test on her. And I remember getting to someplace in Kansas and calling my parents, and we spoke for about 15 minutes, and then my father said, "Oh by the way, Fran, you're pregnant."
There you go.
So anyway, we went to Palo Alto, and I started doing the same type of thing I had done for my thesis of electro production experiments at SLAC with Perl’s group, and we were also involved in the colliding beam that was, Burt Richter was putting together with his group. And we slowly sort of merged. I worked for a couple years, I think, on electro production, but we basically merged into the colliding beam work. And I would say by 1972 or 3 we were mostly doing that.
And of course, the big thing that happened that we called the November Revolution was the discovery of the Psi. And that was a very interesting business. What had happened is we were running the machine in 100 MDV steps in the center of mass, and plotting out the total cross section. And it was interesting because it, unless initially we could only go up to I think 4.8 GEV, but we were seeing an excess up there that wasn't understood. Compared to the basic support model theory.
And Roy Schwitters, who was one of the leading contemporaries of mine in Richter's group, was looking at details of the runs we had done. And he found two runs at, let's see it would have been around 3000 MEV, 3 GEV, center of mass, that had anomalously high rates. And I remember a discussion we had in the cafeteria at SLAC where we got together with James Patterson, was one of the leading accelerator physicists at SLAC. And we asked him, was it possible that the luminosity had suddenly increased by a fairly large amount? A factor of two or three, during these runs. And he said no, the physics of the situation was that that was not possible. So we had this mystery of why we had these runs.
And when you say "runs," Gary, what does that mean?
Oh, well you run the experiment for some length of time, like an hour or so forth, and then you start a new run, keep running it, okay? So it's just a division of the data that's driven often by just the computing convenience of not having too-big files, things like that.
And Gary, just to step back a little bit from the day-to-day, can you describe some of the major research questions that Burt and the research group were after in those years with these projects?
Well, that was the interesting thing, and I mentioned it in this article I sent you.
The proposal which I guess I was at SLAC at the time. I think I was a signatory of the proposal. Listed a number of things that they wanted to do. Measure total cross sections, which in fact we did, but the other things were like measure the form factors of baryons and pions. And it turned out that those things were, first of all, very difficult to measure because the rates were so low, and secondly not of any great interest. And the final thing that was on that list was to search for new leptons. And that was by the collaborations basically considered a joke. They didn't think that was a serious proposal, but that in fact was the one proposal that we followed actually to the letter of exactly what we said we would do.
So the, you know, finding exciting new physics wasn't anticipated at the time that we started the colliding beam work. So it was decided on one weekend, I think on November 10th, 1974, that we would run the accelerator in small steps of energy and see what happened. And of course what we discovered was that just by changing the energy by 2 MEV out of 3,000, or 1 MEV out of 1,000, you could get factors of 100 or even 1,000... I think it was 100, times more rate. And it was just an obvious thing. One of the, we were using spark chambers in those days, and the spark chamber gives out a pulse that gets picked up, and would get picked up by the speakers in our control room. So you could hear when we triggered. It was, you know, "click." And as we were running on the peak of the Psi, it was going, "Click, click, click, click, click."
And I remember Pief Panofsky, who was the director of the lab, came down and was watching this. Because you could also see it on the screen. We had pictures of the events coming up on the screen, and we were used to having one come up every few minutes or something like that. They were just coming up, "Click, click, click, click, click," like that. And he looked at it and he said, "My god. What we've been telling people all these days is actually true." What he meant was that E+/E- was the way to do some fundamental physics as opposed to proton collisions, which was the sort of standard of the day.
So it was a very exciting time. We in 24 hours, we took the data, analyzed it, wrote a Physical Review letter, and submitted it sort of the next day. Actually, you couldn't submit things that fast those days. There was a meeting going on, and we gave it to one of the Brookhaven people who was at the meeting, and he took it back to the headquarters of the Physical Review, which was near Brookhaven.
The other thing which is, of course, had happened is that it turned out that Sam Ting had actually discovered the same thing at Brookhaven, the Psi, by doing proton collisions, and was in the process of checking his results because they were so extraordinary he wanted to make sure that was the state. And he found out through the news of discovery of the Psi, just spread through the world overnight. And he discovered it over the weekend. And went into Panofsky's office Monday morning and said, "I have some interesting news for you." And told him about that. And Pief said, "That's interesting, we have some interesting news also." (laughs) And so we had a joint seminar at SLAC that day. Ting was obviously unprepared to do this, but he gave a SLAC talk. One thing I was going to mention, moving back just a second. That paper, every number in that paper turned out to be wrong. But it was still worth a Nobel prize.
(laughs) When did you find out how wrong the paper was?
Well, as we re-analyzed the data. I mean, first of all, we got the energy wrong because the machine had not been calibrated accurately enough. And it turns out that if, let's see... The resonance was above... I'm not sure about the answer. I think we would have found it faster if we had the right calibration in the energy. So we recalibrated. The energy was off only by 10 MEV, or something, but it was enough to get us over where we had done the previous results.
And Gary, just to be clear, the work was submitted for Nobel consideration with the wrong numbers? Or they were corrected before--
No, well, first of all you don't apply for a Nobel Prize.
No, I didn't mean like that. I meant that the Nobel committee considered the work with the wrong numbers?
Well, I'm sure they didn't, you know, we corrected the numbers in later publications, but the fact that we got the energy wrong and a few other little details wrong were, I think, completely missed by the community, but I realized that afterwards as we were going over, doing the corrections and whatnot, that we missed a few things.
But anyway, so that led to this really explosion of new physics. The reason we called it the November Revolution, first of all of course, it's an obvious takeoff on the Russian Revolution. Was that several things were coming together. First of all, this discovery really cemented the quark model as being real, and not just a mathematical artifact, which I think Gell-Mann thought it was when he first proposed it, and many people questioned is it real, or is this just some mathematics that turns out to be modeled in this way? And seeing a quark that was so heavy, the quark was so heavy, that you could build a non-relativistic model and understand exactly what was going on. A potential and so forth. And it was so much larger than the scale of the strong interactions, which was around 100 MEV, that everyone at that point understood that the quark model was really real.
Gary, I want to ask, as the project was in its conceptual stage, right? Not necessarily by the time you had joined, but maybe even before that. Was your sense that even from the beginning, the project was conceived to answer these fundamental questions? Or was it more that these fundamental questions sort of presented themselves as it became clear what the experiments were yielding?
Well, the idea that you discover a fourth quark, I don't think was, or new leptons or whatever, was in anyone's mind in particular. But the idea here was that you're colliding fundamental particles. Electrons and positrons, which are point particles. As opposed to colliding protons, which have some structure, and so the analogy, and I'm not sure exactly at what timescale it would come in, was that the colliding protons was like colliding two watches and trying to figure out how they worked from seeing all of the debris from the watches. Whereas E+/E- was a very fundamental process, where you're colliding an electron positron into sort of purer energy, and out of that pure energy comes, presumably, a fundamental physics.
But as I said, I mean people thought the idea of a new lepton or initially, even though there was some theoretical interest in a fourth quark, it wasn't really on top of peoples' minds who were doing these experiments. Or when they were sort of being proposed. It all became fairly obvious afterwards. And in fact, what came out of this was a clear understanding that what was going on is that we were producing quark/anti-quark pairs in these collisions, and then seeing these quark/anti-quarks form hadrons.
And one of the key experiments, key analysis points, that was picked up by one of our group members, Gail Hanson, who did analysis, was that we were actually producing jets of particles that corresponded to these primordial quarks that were being produced. And--
Jets as opposed to what, Gary? What else would they have been?
As opposed to, say, a fireball. There was a theoretical paper that was written by Bjorken and, what's someone else it may have been?
No, I think it was Bjorken and... again, I'm dropping a name, but I can go back. That was saying, we really don't know what's going to happen in E+/E- annihilation. Are we going to see jets? Or are we going to see a fireball of particles? And so this was-- In fact, they proposed a way to say that. The metric. We used the metric that they had proposed. It was, I can look it up in our paper. So, and then it was very interesting, because Gail had been very systematic of making plots of everything, even if they shouldn't be very interesting. And one of the plots she made was of the azimuthal distribution. Now, you'd expect that to be isotropic, and it wasn't. And Roy Schwitters looked at it. (laughs)
And why would you expect it to be isotropic? What suggests that it would be?
Oh, just the symmetry of the situation, right? I mean, you're colliding two things in a line. You don't expect there to be an up and down as opposed to a horizontal. But it wasn't. And Roy Schwitters, who had been really studying a lot of accelerator physics, took one look at it and knew exactly what was happening. What was happening was that our beams were becoming polarized. And this was a sign, what we were seeing was a sign of that polarization. And that, the distribution that we were seeing, is exactly what you would get if you collided two-- If you produced two and a half particles. So that again sort of confirmed the whole thing, and Roy was the lead author in a follow-on paper there.
One of the interesting things we did in the mark two, that's never been done in large particle physics beyond that that I know, is that except for the first few general papers that we published, this is in the whole mark one, mark two series of experiments at SLAC. We would list the primary authors first, and then everyone else in alphabetical order. And that was very nice, because it made it so easy to write recommendations for postdocs and so forth, and say, "So-so et al, so and so et al, he did these papers." And it was sort of obvious.
And in fact, I'm not sure whether Martin Perl would have won a Nobel prize if we hadn't done that. Because if someone else's name was first, it wasn't going to be so obvious outside the group where it came from, but it was Perl et al. And in fact, when I was... Perl very graciously invited me and my wife to the Nobel ceremonies, and there I was talking to one of the Nobel committee members, and I said, "This, as far as I know, is the first time that two Nobel prizes have been awarded for the same experiment." And he was shocked to discover that it was the same experiment. They hadn't realized. Of course, these came almost 20 years apart. Richter got it two years after the discovery of the Psi, and Perl got it 20 years after the work on the Tau. So that actually helped a lot.
And so anyway, so Gail became well-known for this work on the jets, and so forth. So let's see, we're... There was just an explosion of work. The years '75, '76, '77, we were just discovering all of these new particles. Discovering the jet structure. And, you know, putting all of this together. And so it was just a very exciting time.
Gary, was your sense that by the mid-1970s the grand plan that Panofsky had put together some 10, 15 years earlier, was it sort of all coming to fruition at that time?
Basically yes, yeah. The idea of using electrons as probes has been well-justified by what happened. Let's see. Going on from there, we rebuilt a detector. It was interesting dynamics there. I think it was around 1975-76, that Burt decided that we'd probably discovered most of the interesting stuff. And his group worked just on the detector, and Perl's group and the LBL group would continue running and getting all these dull details.
And Gary, how involved was Burt on the day-to-day? Was he right there with you every single day, or was he managing other projects at the same time?
Oh no, he was there all the time. You know, this was, the other thing that's hard to understand in terms of modern experiments, is that the Psi paper had 30-some signatories. Today, if you propose an experiment with 30 people, you would never get by any committee, it's clearly too few. And in fact, some of those people had already left. So the thing was all SLAC and LBL. Later, some user groups joined, particularly when we got to the SLC, but it was all a local group, and so it was very tight, and this was Burt's main interest. I think he became at one point research director or something like that. But that was a little later on. And eventually became lab director. But no, he would be down in the control room seeing what was going on. He was... And all of us were working fairly closely together. So anyway, what happened was we actually made a significant discovery. We discovered a resonance right above the charm threshold.
What year would this have been?
I can, I did a little research last night. Let me get it. We published the paper in 1977. It was submitted on June 1st, 1977.
So it was right above charm threshold, and so what was being produced entirely was pure DD bar mesons. In other words, mesons which have a charm quark in it. And so it was a great factory, and in fact experiments were done on that and it produced a great deal of very clean information about the charm meson system. In fact, the Chinese continued for many years with basically a dedicated facility to study these properties. So Burt later realized and admitted that it was a mistake for his group to pull out of the experiment at that time. Anyway--
Why? Why was it a mistake?
Oh, because they missed this discovery and so forth, and a chance to work on the data. In that paper, in fact, I was looking at that paper last night, we'd given an acknowledgement to some of the key people in Burt's group that were helpful at SLAC, even though they weren't part of the experiment at that point. So we rebuilt the detector as a more modern detector. Basically, these detectors were called the mark one and mark two only after the mark two. The original detector was called the SLAC LDL detector. And then we discriminated them as mark one and mark two.
So we built the mark two detector. It had much better vertex. It started using drift chambers as opposed to spark chambers. And my assignment in that actually was the muon system. And I designed these aluminum extrused, triangular aluminum extrusions, so that you would have one triangle pointing up, and next to it one point travel-- one cell's pointing down. And so most particles would go through two cells, so you'd get more resolution from the readout. This wasn't an original idea. I picked it up from somewhere else. And I had a lot of fun with that in the sense that it was the only time that I designed an electronic circuit that actually got produced. And then it was so simple, that I figured I could do it myself.
And like most physicists as opposed to engineers, I over-designed it. One of the problems with proportional chambers is that you have electronics that tends to oscillate. And I was determined that mine would not oscillate. So I built in a tremendous amount. And this was okay, I mean it meant that the thing was slow, but in a colliding beam, you have to wait for the beam to come around again anyway, so it's okay being slow. And I remember at one point, I was testing these on cosmic rays, and our chief electronics engineer came by, and I said, "Let me show you something." I disconnected the gas, and I disconnected the high energy. And it still continued to run. And he looked at it and he said, "Well, it won't do that forever." (both laugh) It had so much capacitance that it would just run by itself, it was like a battery.
And when you say, Gary, that the product was mass produced, in what applications would we find it?
No, just in our application. I mean, we had hundreds of cells, so reproduce that. Anyway, so we ran with the mark two. Again, some of the significant things were, we did the first measurement of the Tao lifetime. Not the first measurement. The first measurement that produced a non-zero answer, as opposed to a number. And I was actually surprised, as I went over a few papers last night to remind myself of all of these things, that I was actually the lead author on that. So I had forgotten that. The B lifetime was measured well, things like that. And, you know, quite a bit other detailed physics.
The mark two detector started at SPEAR. It got moved to PAP, which was the... SPEAR was capable of getting up to I think 6 GEV in the center of mass. Maybe it was slightly higher. PAP was running usually at 29 GEV in the center of mass. And again, there were no major discoveries, but lots of detailed physics came out of that work. We were of course hoping to discover the-- Well, one of the things that you could do there was study the B meson system. And again, there was a resonance right above the threshold and you could clean BB bar pairs, and that lead to a whole industry. And later on, which in fact I can comment on, factories both in Japan and at SLAC, just to study the BB bar system.
But the next thing that was getting excited was Burt's project to build a linear collider. And he wrote a paper earlier, pointing out that eventually just using scaling laws, eventually a linear collider would be needed to do E+/E- annihilation. In other words, there had to be some energy at which it would work better, just in terms of how the various things scaled.
Gary, do you know when Burt committed to this project, roughly what year that was?
No, I could probably do a little research on it, but I don't know it.
It would probably-- Like early 80s?
Let's see, let me... Probably. We got it running in 1989. That was the first papers. And that was later than we expected, so I imagine that he was thinking about this and running it probably from the early 80s. I remember he was on sabbatical at CERN when he wrote this paper.
I can look that up. But so what this was not a true linear collider in the sense that he was using the SLAC linear accelerator, and then splitting the positron beam from the electron beam, and bringing them around in one arc. Well, if you only bring them around in one arc, you don't lose so much energy. You may lose an MEV or so. It's only when you circulate them that you have-- In other words, the reason-- Well, let me go back. The reason why a linear collider would eventually be better than a circular collider was that the radiation, the synchrotron radiation from a circular collider would eventually cost more in the energy that you have to put into it, the RF power, than in the linear collider, where it's just linear, whereas I think it was fourth power of the energy than the circular collider.
So anyway, so this was the project, and in order to make it work, you had to have these beams be extremely small. Otherwise you wouldn't get enough density in the collisions. So that got started. We said that we had the mark two detector there. Critics said, well the only part of the mark two detector that you've brought over was the nameplate. We actually rebuilt everything for it with better technology. And we got on the air just before LEP turned on. Now LEP was going to, in terms of the physics, LEP was going to wipe us out. We knew that. But we were just slightly ahead of them.
Why? Why was LEP going to wipe you out?
Oh, because their luminosity was much higher.
And there's a direct correlation there between--
Had handfuls of events, and they would have hundreds and thousands.
Right, so you're saying there's a direct correlation between luminosity and how much there is to uncover?
Yeah, I mean the more events you have, the more precision you get.
But we were able to do the first-- One of the things that I found really fun, I was with Jonathan Dorfan, were the two co-spokespersons of the SLC part of the experiment. The mark two at the SLC. And we had to find the Z, so what you would do is you would take a guess based on theory, and whatever extrapolations you had, and you'd run and you'd measure the total cross section there. And you knew from the physics, you knew what the Z total cross section had to be. So you would then guess, so that you do the first measurement, and you see you're not there, but is it a lower energy or a higher energy? And then you take a guess, and by the time you've measured two energies, you have a pretty good idea of what's going on, and then you fill it in with a stepping around and getting points of different energies.
And we did that, and got that published a little before LEP-- Some of the stuff was just a few weeks before LEP got started. And the number we came up with for the Z mass was right on the actual number that LEP had, very high precision. LEP's numbers were very high precision. And the other thing that-- So we mapped out the properties of the Z, and the most significant one, and we were a little lucky there, was the number of neutrino families. You could determine that by the, officially by the width, but it turns out it's actually the easy way to make measurements. The width of the resonance, since it would go into all these different pairs including the neutrino pairs, you could count the number of neutrino pairs by the fact that the width was larger, or because the width was larger, the peak was depressed. And we got that published early.
And the thing that we were a little lucky on is that we measured a number that was slightly smaller than three. The number 2.8+/- something. So we could claim at some level of significance that there were only three neutrino families. And of course, LEP measured that to probably two orders of magnitude more precisely than we did eventually.
But it was significant that you started earlier? That did matter?
Oh yeah. We expected to be there, you know, these things always take longer than you think. And we expected to be there quite a bit earlier than that. But it turned out that we were almost at the same time. But it was just a lot of fun. There was a lot of new physics to do there, and you at least got a good look at the physics first.
At that point, well, at that point a couple things happened. One was that I got an offer to come to Harvard, and Roy Schwitters was at Harvard at the time. I think that probably helped. I was a little surprised, I said, you know, "You didn't even interview me." He said, "Oh well, we know you." So I was very tempted to take it for a couple reasons. In fact, I did take it. One was I wanted to do more teaching in a significant way. Now, I was teaching. What had happened was that, you know, SLAC is separate from the physics department at Stanford, but SLAC people, professors, can teach voluntarily in the physics department, and I did that, some graduate courses. And once I knew I was going to Harvard, I taught at least one undergraduate course, which turned out to be a fiasco, but that's another story.
Gary, I did want to ask you about that. I know that there is that separation between the physics program proper and SLAC. Aside from your own initiative to want to teach courses, did you feel sort of generally connected to the physics program, in terms of, you know, did professors come over to SLAC who were interested in what you guys were up to? Or was it generally, did it feel more separated than connected?
It was in general more separated than connected. Now, there was some old differences, or whatever you might want to call it, that you know, went back to the founding of SLAC. And there were some people in the SLAC physics faculty that worked at SLAC. Stan Wojcicki was, for instance, one of them. He actually didn't do much at SLAC, but he had an office there. And oh god... The guy that won the Nobel prize with Lederman and Steinberger. Schwartz. I'm trying to get his first name now. Anyway. He was on the SLAC faculty. So there was some connections.
Melvin. Melvin Schwartz.
It wasn't much going back and forth. Very rarely would anyone from SLAC go to a talk on campus. And equally rarely would someone not affiliated already, or who already had an office at SLAC--
-- come out to SLAC.
So I wanted to ask, in terms of--
-- there was a separation, and even though I was doing some teaching, I didn't feel very connected to it.
In other words, I wasn't in on the faculty meetings that would discuss policies and things and so forth. So I wanted to do more teaching.
So essentially, SLAC was a world unto itself? You didn't feel part of the Stanford academic community, more or less.
I mean you could, and we did. I mean, I advised freshmen, for example, like that. So you could participate. And in fact, I was on the academic senate at Stanford. Because SLAC, being part of Stanford, had one or two delegates to the academic senate.
And I'm curious, Gary. You're a--
So I had some connections, but I wasn't part of the department.
And you're an employee of Stanford, right? Your paychecks say Stanford, not SLAC.
Our paychecks definitely said Stanford.
But that's the way all of the National Labs work. You're not government employees, you're contractors. So Brookhaven, Fermilab, and so forth are run by management committees that are often multi-university collaborations. There's actually a bidding process for the whole thing. But SLAC was the one national lab that was most closely connected to its university. I mean, it was definitely a Stanford thing. It was on Stanford land, and so forth. But we were separate. We were as separate as, you know, I forgot what they call it. At Harvard we call it the Faculty of Arts and Sciences. And then we have the different graduate schools for Law and Medicine and so forth. SLAC was its own school.
It wasn't part of whatever-- I don't know the Stanford terminology at this point, but it wasn't part of the Stanford academic system.
Gary, I want to ask you another question because it's relevant to the timing of the Harvard offer, right around '89 and '90, and that is, you know, you often hear about a Golden Age in fundamental particle physics, and you know, one of those-- It's hard to determine exactly, you know, how to bookend that age, but a lot of people do say that it's, you know, it's right around the late 1980s and early 1990s, and particularly after, you know, SSC sort of fell apart.
And so I'm curious from your vantage point, when you're considering this offer from Harvard, if part of it was that some of the most impactful work that SLAC was able to do circa late 1980s, was already behind you? In other words, in looking ahead to the 1990s, what might have been on the horizon in terms of anything that would have rivaled what was discovered in the 70s and 80s in terms of what SLAC was looking to do?
Well yeah, from my point of view, that was the other-- One thing was teaching and being part of an academic department. And SLAC actually, Stanford actually said, "Well, if you want an appointment in the department, we can do that." But the other part was, I felt that the academic constraints that I would have by staying at Stanford were not as good as the opportunities I could have at Harvard.
And there were two aspects of that. One was that I didn't see a big future for SLAC. Now, what had happened was I had been working for a number of years on the idea of building this B factory, based on an idea from Pier Odone that it should be asymmetric for a couple reasons. And it just wasn't getting very far. And the thinking, the betting odds I would say, was that it wasn't going to happen. What actually did happen was that Burt pulled it off. He had connections and somehow got it through Congress and so forth. So it did happen, and it was a successful project. So I didn't think that was going to happen.
So that was one of the things. So I wasn't in any position... There was another group led by Marty Breidenbach and so forth that was going to continue the work at the linear collider for some time, and I just didn't see much opportunity at SLAC itself. And Stanford was being supported by... The Stanford physics faculty was being supported by the NSF, which wasn't as good as being supported by the DOE. And Harvard was a DOE institution. So--
You mean that the funding was more stable at DOE, or it was bigger? What does that mean?
It was more stable and easier to get. The NSF works best on sort of small science. In other words, they're keen to sort of smaller projects. Whereas the DOE likes large projects and so forth. So I didn't know, going to Harvard, exactly what I would do, but I thought I would have much more freedom in terms of choosing what I wanted to do. So I was wrong about the B factory, but things worked out for me fairly well anyway.
So what happened, I got to Harvard and the situation there was that Roy Schwitters was in the process of leaving. In fact, by the time I got there-- I delayed getting there until I had finished the stuff I wanted to do on the mark two at SLC. So when I went there in 1990, I think I started in the fall of 1990, and Roy Schwitters had already left Harvard to be the director of the SSC. And as a result, there was a big hole in that there was no senior person working on the CDF, leading the CDF group at Harvard. There were two junior faculty members, and I'm not sure exactly of when their dates of appointment were. They may have come slightly after I came, or it may have been very close to it. Melissa Franklin, who was actually one of my graduate students at SLAC, and John Huth, who is one of the leading young physicists on CDF at Fermilab.
And what was the state of play of the CDF group at that point?
Well, it was a sizable group. It had graduate students and, you know, they were doing good work. So I saw sort of a vacuum there that I felt I had to fill. And I did that for a number of years. I took over one of, at least one, maybe only one of Roy's graduate students. But I was never very satisfied by it. I felt that with teaching obligations and not having a history in CDF, that I just wasn't in a position to contribute very much.
But you only came to that realization after you had joined the group?
Well, I suspected it at the start, but you know, I was just trying to be a good citizen and help out. And I didn't know exactly where I wanted to go anyway. So it was a good thing to work on, but I never enjoyed it. I found that it was too big, it was too inbred. You had to spend more time than I could afford to spend on it. In fact, I decided that if you tried to read every paper that had your name on it, that was the only thing you would have time to do. (laughs) So anyway, so I did that for a little bit. So my name is actually on a bunch of CDF papers, but I can assure you that the majority of them I have never read.
Anyway, what happened was that we had a new assistant professor come into the department, Sanjib Mishra. And he had worked on neutrinos. In fact, let me go back a step, because I left out something that I enjoyed very much. In 1982-83 academic year, I took a sabbatical from SLAC. (laughs) My brother in law I remember said, "Let's see, are you teaching? No? You know, what exactly are you taking a sabbatical from?" And that was a good question. But anyway, I got an appointment at CERN, they would pay half my salary and SLAC would pay half my salary. And I joined, again, it was one of those deals where I could join any group, and I decided to join Jack Steinberger's group. And work on a neutrino oscillation experiment.
I've always been fascinated by neutrinos. And this was a great experiment. We didn't find anything, because we were looking in the wrong place, but it was six people and we did it all in one year and published the paper, wrote the paper and published it in that year. And the limits that we set lasted for decades. No one had done better. So I enjoyed that very much. So that was my first introduction to neutrino physics.
Anyway, Sanjib Mishra joined the department, and was talking, had been working with the neutrino group at Fermilab, and had some project. I don't even remember the details of it now that he wanted to-- And so we put in a joint proposal. Joint in the sense it was Sanjib and me, to do this experiment, and we got turned down by the group at Fermilab. So we're looking around, and the theorists at that time were very strong on the fact that the dark matter of the universe had to be 20 EV tao neutrinos. And they were all very sure of that. And there was this experiment at CERN to look for those.
So we joined that experiment. It was called the NOMAD experiment. And it was a beautiful experiment. It had what we called an electronic bubble chamber. And by that what we meant was, it wasn't a bubble chamber, it was drift chambers, but they were very low-density, so we had the density of liquid hydrogen, like you would have in a bubble chamber. So it was a very precise measurement and it all worked very well. Many interesting papers came out of it. We were looking for the wrong thing. There was no 20 EV tau neutrino. The tau neutrino was much lighter. And so two things happened, I'll talk... So anyway, a lot of good physics came out of it, and it was a nice experiment.
The other thing that happened that probably is, in the community may even be my best-known thing, was that one of the people in the experiment, Americans, was Bob Cousins, from UCLA. And Bob was very strong in statistics. And he taught me all I learned about statistics, in fact. Including how to do an Iman instruction.
And one of the questions that I was kind of interested in was what happens if you're doing an experiment where you're searching for something that may not exist, and you have some background and those backgrounds will fluctuate, and you'll measure a number of different, say, mass states that you're searching for. And some of the time you'll get less background than you predicted. How do you set a limit on that? And in the extreme case, what happens is that, say you're setting a 90 or 95% limit. Those are the typical limits you set, upper and lower bounds. Well, in this case it would be an upper bound. But what happens if the fluctuation is so large, which happens if it's 90%? Well, it would fluctuate both ways, so it would be maybe 5% of the time, you would get a limit that would exclude that the particle at any level of its production. Which is much better than your sensitivity. And so it didn't seem right. And so one of the questions I was interested in was, well how do we do the statistics on this, because we're likely to be in that situation. In fact, we were in that situation. And--
Did you know you were in that situation then, or only looking back?
No, we suspected that we might be in that situation. We knew after a year. I can also tell you another interesting story there. In fact, I should. I'll get to that. We knew we weren't seeing anything after the first year, but you know, we wanted to take more data. So I was thinking about that and I had learned from Bob how to do a name on construction, and the point in a name on construction is that you have some choice. In other words, where you put the limits. In other words, if you're trying for an upper limit, you can put them starting at zero, going up, if you're trying to get an interval you can, for instance, do shortest distance or you can do central value, which is the common one. In other words, the most likely value is in the middle, and you go equal distance on each side in probability space.
And what had occurred to me is, why would you rule out the most probable value? For instance, in the case I gave you, which would be zero. And so I proposed to Bob, why don't you use maximum likelihood as your organizing principle? In other words, you first take a slice at the maximum likelihood and then you take another slice and so forth. And build up this construction that way. And we went back and forth and did some calculations and realized that this did everything right. Everything we wanted to do.
And so we wrote this paper and Bob insisted on putting my name first because, well, he came up with several reasons. (laughs) One was, it was my idea. But he said, "Well, it's like, you know, you put a student's name first and the professor's afterwards. And you've been my best student." So anyway, it became known as Feldman-Cousins. And it solved a problem that had been sort of vexing particularly the high energy community for a long time.
So a couple things happened. One was, let me tell you two stories. One was that I, after we got this paper ready and submitted, I had some friends, at least one friend in the statistics department, so I proposed that I would give a seminar to the statistics department. And this of course was a very funny thing, because it was clear that I was a complete amateur. And I kept explaining things and they would say, "We know that, we know that." You know? So I got to the end of my thing, my presentation, and the response from the statisticians was, "Well, what you've done is the standard thing to do." And I said, I was happy to hear that, I'm glad we did the standard thing, but can you give me one example where it's ever been done this way? And the answer was no, they couldn't think of any. And they explained to me why it was the standard thing. They said, well you're making a series of hypothesis tests, and we know that the most powerful hypothesis test is with maximum likelihood, so of course you would use maximum likelihood to do it.
So I went away, and went back to what's considered sort of the bible of statistics classically. So statisticians told me you should look at some more modern work sometimes too. Kendall and Stuart's book. Later after Kendall died, it became Stuart and Ord, I think. And they renamed it Kendall's Advanced Theory of Statistics. And instead of looking at the chapter on setting confidence limits, I looked at the chapter on hypothesis testing. And sure enough, there's about a page and a quarter right at the start of that chapter, where they lay out exactly what I had done.
And so we wrote a note added in proof to our paper saying that, you know, afterwards, we discovered that this had been around for 50 years, but no one had used it. It was almost 50 years when they first did it. I went back to old copies of Kendall and Stuart. Harvard has a good library for that.
So anyway, so that was one thing. But the other thing was that it was clearly a need for this in the community, and the particle physics group, the particle data group at LBL, which is the one, I don't know if you're familiar with it, it collects all of the data. And sets some standards. The person who was sort of leading that, Don Groom, realized the significance of this, and caught on immediately, and popularized it.
So it very quickly spread and became the standard way to do statistics when you're near a physical boundary. I mean, this gives-- if you're away from all physical boundaries, and you have gaussians, this gives that same answer. But the question was, how do you do it when you're near a physical boundary? And it caught on, and it's now used fairly universally in at least high energy physics and astrophysics. And so it's my most-cited paper. In Inspire, it has 3,200 citations. In Google Scholar, it's over 5,000. And some of those citations are from people who are criticizing it, or suggesting, you know, here's a case where it doesn't do so well, so we make a little tweak and do better. And we never, Bob and I were never impressed with that, because it's a whack-a-mole problem. If you make a little tweak here, the problem's going to show up somewhere else. So we didn't think there was any way to improve it. And so anyway, that was the one thing that came out of it. That was the other thing that came out of the NOMAD experiment that has lasted.
Let me go on, then. So that was our first experiment, my second experience in neutrino oscillations. And of course, there were these measurements being done on long baseline experiments, mostly in atmospheric, not in accelerator, that were getting mixed results, and I wasn't very convinced of them. It was hard to get convinced. Until 1998, and in 1998 the Super-Kamiokande results came out. And they were beautiful. You looked at them, and there was no denying what was happening. There really were oscillations. It was very clear. And at that point, I said I've got to work on this. And I joined, both Sanjib and I joined the MINOS experiment that was in preparation.
Where was that experiment happening?
It doesn't happen for another -- So this was in 1998 that we joined. Again, I can look it up. It's going to be into the 2000s. What happened was, I'll tell you the story in a minute. First publication came in, submitted at the end of 2005. It was published in 2006. So what happened was that there was a shoot out. This was before I got involved. Fermilab, there was I think a proposal, and the Fermilab committee said that it wasn't sufficient and invited proposals for long baseline neutrino oscillation experiment. And there was one from the Sudan group to build, to use the Soudan detector. I think in Northern Minnesota. The lowest level of the Soudan mine. There was one other detector, and I forgot, it may have been to use the IMB detector. I don't remember, I can again look it up.
And the third was a one-person proposal from Stan Wojcicki to do it at SLAC. To send the beam over to SLAC. Well, the decision-- So the committee said -- Oh no, I think it was the macro group, I'm sorry. The macro group, and I forgot where they wanted to have their detector. The committee said, "Why don't the three of you get together, three groups get together, and come back with a joint proposal." And so that's what happened. Stan Wojcicki was elected to be, or selected to be the spokesperson. And the decision was that, we'd build a new detector, but we'd do it in the Soudan mine.
And the reason for that was, well, maybe the detector won't be ready when the beam is ready, and so we can use the Soudan detector, which is a one kiloton detector, instead of this new detector, which was proposed to be 7.5 kilotons, but ended up being 5.4 kilotons. And the irony was that the detector was built and ready to take data two years before the beam was. So the beam wasn't ready until around 2006. The detector was actually finished around 2004 and started taking data on cosmic rays.
So we worked on it for about four years before it got started. I had some graduate students who did very minor experiment at Fermilab and beam time in order to finish their theses on time. And so the MINOS experiment was this general purpose experiment. We really didn't know much. In fact, a lot of it was based -- One of the problems was that when it was designed-- Well, actually there was a shoot-out. It was an interesting shoot out. There was not only this proposal, but there was a proposal from Brookhaven to use the Brookhaven accelerator to do an experiment there, by using an off-axis beam, which was an interesting proposal because you get a-- If you use an off-axis beam, you get a narrow band beam that you can tune its energy to by the angle.
And this was before I joined the MINOS detector, I was on the committee that decided the shoot-out. It was led by Frank Scully from Columbia, the Scully committee. And the reason, the thing that we didn't know at the time, is what the mass of this oscillation, the mass difference where this oscillation was. Because of Soudan, the Kamiokande numbers were quite a bit higher than the Super-Kamiokande numbers. Super-Kamiokande was right. But the planning was for the higher numbers.
And the one of-- There were a couple of reasons. One was a complete red herring, the other was significant. Why Fermilab was chosen. And that was that it had a better range of energies, of mass squared values that it could accommodate. And it turned out that that ended up being a crucial thing, that the Brookhaven would have missed the peak by some distance. So let's see. So anyway, MINOS was very good at detecting muons. It was a basically an iron scintillator sandwich. It was not very good at detecting electrons. It did the sort of pioneering work there, but the results were not terribly significant. It did see electrons and identify them eventually. So actually, starting even before we started taking data, interestingly enough, on MINOS, we started thinking about another detector that would emphasize the electron appearance, because it was, I think it was even clear at that time that that would become the important thing.
And we started talking about it in workshops in 2002, and the leader of that was Adam Para at Fermilab. And I think I became the chair of a committee to decide on the technology that we were going to use. And there was a proposal used liquid scintillator and a proposal to use these glass chambers. I've forgotten now what they're called. Again, I'll look it up. That Indian collaboration was working on building. And basically, I was of the opinion that liquid scintillator was going to be a lot simpler. Because these chambers had a history of not working. And the argument was, well, they're better. We have better technology now and so forth and so on. But my experience with active chambers was that they're tricky to use. Whereas scintillator, you just put it in and it works. And liquid scintillator is even easier, because well, it doesn't change too much with time due to oxidation. Because it starts out completely oxidated. So we went in that direction, and Para was really only interested in using these other chambers, so he dropped out. And basically, I became the spokesperson, and I stayed as a co-spokesperson for 11 years. And at that point I was nearing retirement and decided it was time for younger people to take over.
Gary, 11 years suggests that there's something new to say over a long period of time.
Well, don't forget we didn't start running. (laughs) Building this detector was a long process. And we had some hiccups on the way. One major hiccup that cost us about a year was in 2008, Congress around Christmas time just zeroed the project out to save money because of the poor financial situation in the country. That got reversed about six months later, but the problem was that the work crew that was doing all the preparation had sort of dispersed. And so you had to put everything back together again, and that cost us about six months.
So we were delayed for a year there, but there are always other delays. You don't get the funding you want, and actually, what happened was ironic. What happened was that after being zeroed out due to the financial situation, we got stimulated due to the financial situation. 2008, we got stimulus funds that we had to spend in the U.S., but that was okay. But they in fact gave us so much money, we couldn't even spend it efficiently at that time. But anyway, it finally worked. But we didn't start taking data until around 2006 or-- I'm sorry. So it wasn't 2008, when was the date? 2008 may have been NoVA. That was No-- I'm getting it confused, I'm sorry.
That was NoVA. But it was this... Anyway, it... No, that's what I'm discussing. I'm discussing NoVA. I'm sorry. I'm right. We didn't start taking data until 2014. So we had worked on this thing for 12 years, building it and getting it running before we were ready to start taking data. We started taking data February of 2014.
And how soon after, when you were taking data, were you starting to get interesting information?
Well, it's slow. I mean neutrinos don't inter -- We wanted, our initial proposal was for with this technology, which actually... We had started with a sandwich-type technology with rather than iron, some low-density absorbers. And we wanted 50 kilotons. Stan Wojcicki convinced us that maybe we should build what we called a totally active detector, and it would just have liquid argon and minimal separations of the cells, which we did eventually with PVC extrusions. And we proposed 30 kilotons.
We were eventually, again, the financial estimates that we made, we tried to be conservative, were low. And we were eventually told to build a hall for 18 kilotons, but build as much as we could for our budget. Our budget was our budget, build what you can. It must be at least 14 kilotons. Well, we ended up with 14 kilotons. But so the data-taking was slow, but we've been taking data and it moves around. We originally talked about taking data for six years. It's now clear we're going to take data until 2025, which would be nine years. And if there are delays in the DUNE project, it may go longer.
We have competition from the-- The one competition is from the T to K experiment, which is shorter baseline, so they don't have as much sensitivity to the matter effect, which discriminates between the normal and inverted mass hierarchies. But we're eventually going to, probably in a year or so, start publishing some joint papers with the joint fits of the data. And we're getting there. It just takes a lot of data, and in the meantime, produced, you know, other interesting results in addition to the oscillation data.
Gary, I'm curious. Since there's such a long gestation period for this project, have the research questions that you had at the beginning of the project, have they changed over time now that you're sort of right in the middle of collecting the data?
Not really. What we wanted to explore was sort of the standard model, which is like the quark model. It's a unitary matrix, a 3x3 matrix, and has provision for CP violation. We wanted to explore that as well as we could and determine all of its parameters. In addition, of course, we wanted to see if there was anything new or interesting. One of the things we've done is search for sterile neutrinos that could be there, for instance. Or data that would be incompatible with the standard data. Standard model. But so far, all the limits that we have on sterile neutrinos show no sign of them, and the standard model seems to work quite well, and it's just a question of pinning down these parameters.
And again, if the physics, if we're lucky, we can do a pretty complete job. If we're not lucky, we won't be quite as conclusive about the results, and perhaps the DUNE experiment and the hyper K experiment being proposed in Japan will finish the job. But we hope to address, you know, as much of this with whatever precision we can get. And we're mainly still statistics-limited.
What's the dividing point between being lucky and not being lucky?
Well, one of them is the mass ordering. All the evidence in the world is this, what we call the "normal" ordering, where there are three mass states that are combined to make the three sort of flavor states of neutrinos. Two of them are very close together, this is known as a solar oscillation, and one of them is well-separated by about a factor of 30 in the mass squared difference, known as the atmospheric oscillation range, because it was first discovered, of course, in the atmospheric neutrinos that were seen by Super-Kamiokande. So the... I'm sorry, let's see, what was the question? (laughs)
You just, I mean I just grabbed on this interesting idea that, you know, when you're looking at where this is headed, the project will head one way if you're lucky, and a very different way if you're not lucky.
Yeah, so it turns out-- here's the problem. The CP violation. There's two things that separate neutrinos and anti-neutrinos in terms of what you see. One is the CP violation, which reverses for the anti-neutrino. So if you... And that's caused by a matter effect. By an effect of coherent oscillation through the earth and it looks like a CP violation, because the Earth has electrons in it but doesn't have positrons in it. And so electron neutrinos have a change in their coherent forward scattering as they go through the Earth. Sounds incredible, but that's what happens. So that's one effect.
The other effect is the actual CP violation. Now these two effects are approximately the same size. So in the NoVA experiment. In the T to K experiment, the matter effect is fairly minor because it grows with energy, and we're at three times the energy. But the CP effect is the same. So if they go in the same direction, then that's where we're being very lucky, because then it's unambiguous. In other words, if the CP goes in the same direction as the matter, and that's what we were hoping would happen. Our latest results are more ambiguous. And in fact, I'm not even supposed to talk about them, but they're going to be released next week anyway.
Okay. (laughs) Hot off the press.
They've really become quite ambiguous in that. And we're hoping that'll change. The T to K results are less ambiguous. They don't see much matter effect, but they see far too much CP violation.
Which tells you what?
So if you put them together, I think you actually get a coherent picture, but we're not ready for that yet. Anyway, this is a work in progress, and that's what I mean by being lucky.
Now, you have five years. A lot can change in five years, and so I'm just curious. Best-case scenario, most luck, however you want to define it. What does this project look like by 2025?
Well, I hope we'll have sufficient statistics that we can at least pin down these parameters with some reasonable precision. That would be the best thing. Even better, if we actually discover something new. If we discover there's an incompatibility somewhere, which we don't see at the moment. Again, you need high statistics to do that. Or if we find evidence for a sterile neutrino. Or even if our other people at Fermilab are doing short-based time experiments find evidence, true evidence, for a sterile neutrino, that would be very exciting. As you probably know, the evidence for that is very murky at the moment.
And I'm hoping, I don't know if it's going to happen or not, that the neutrino conference starts as a virtual conference next week. And I'm hoping that we'll get some interesting information from the micro experiment on what they've discovered, but we'll see. There's a longer range experiment that should be decisive that's now being planned for Fermilab.
Well Gary, now that we're up to the present day, I think I want to ask you for the last part of our conversation sort of a few retrospective questions about your career in total, and maybe a question, a forward-looking question about, you know, things to look for in the future. And so the first is, the transition from particle physics to neutrino physics. Do you look at this as sort of a natural, intellectual transition for you, or was this a moment of opportunity to reinvent yourself as a physicist and the kinds of things that you wanted to work on for, you know, the second half of your career?
Well, I'd always been interested in it, actually. And when the opportunity came, I was very happy to take it. I mean, it's all particle physics. It's just a different branch. I was never that interested-- I mean, my whole career has been dealing with either electrons or neutrinos. Both of which are fundamental particles, point-blank particles. As opposed to dealing, except for the short time I was on CDF dealing with proton collisions. So I think it was a natural evolution. It was a change, but the opportunities arose, and I felt the more interesting opportunities were going to be in neutrino physics, and that was right. The one thing I didn't discuss, by the way, was the situation with the SSC and with the LHC.
Maybe I could say a few words about that.
I did work on the-- I left it out of my description. I worked on the SSC. I started working on that with a group of West Coast people when I was at SLAC.
Who else was in that group?
Well, George Trilling, I think was one of actually the leaders in the experiment. And the University of Washington was involved. And what we were-- What I got assigned to do by George was worry about the muon detection. So, you know, once you've done muon detection, I guess you become a muon detector expert. So we worked on that, and then the, as you know, in what was it? I forgot the date now. Was it around 2003? I think. The SSC got canceled.
No, earlier. '93.
'93? Oh okay, yes, you're right. It's, I'm sorry, it was 1993. So I continued working on it with some people on the East Coast. So that got canceled. My take on the whole situation, the SSC was really the right experiment to do. It was a better experiment in the lab. The detector we were proposing, I think was better than the lab-- I don't mean that, I mean LHC. The LHC experiments. But what happened was though I have great respect for the director, what's his? I just mentioned his name earlier.
Roy Schwitters? Roy Schwitters.
Roy Schwitters. So I have great respect for Roy Schwitters, but he ended up getting in over his head. The head of the DOE, which was Admiral Watkins at the time, never had any great respect for him. They didn't have things very much under control. He got bad advice from the Texans he was dealing with. I think the Drell panel, which at one point considered whether they should downsize some, gave him bad advice. And in the end, they couldn't defend the cost estimates. There was a worry in Congress that-- Two things happened. Three things may have happened. First of all, the SSC started with Texas being politically very strong. It ended with Texas being weak.
So that was a political thing. The second thing that happened was that the economy changed. And there was a great desire for Congress to at least show that they were doing something about it, and canceling the SSC was an easy thing to do because it didn't affect so many people.
Canceling a NASA project where it was spread over the whole country would have been a whole other thing. Anyway, and the third thing was that the SSC wasn't able to sort of guarantee that they could meet their financial goals. I mean, they were talking about a $4 billion project, that there was worry that it would be a $12 billion project and so forth. Now, my feeling is that you know, before Roy Schwitters was offered the job of director, Burt Richter was. And if Burt had accepted that, I think the SSC would have gone through.
Burt had enough gravitas and enough political connections, that I think he would have been able to pull it through.
Do you know if Burt ever seriously considered the offer?
I think he seriously considered it, but he wasn't so interested in it. His real interest had always been building a linear collider. And that was his dream, and that was what he wanted to work on. And you know, it didn't happen. It's not going to happen, as far as I can tell. There's not much justification for it at the moment, because it would work at mainly at lower energies, and the LHC hasn't found anything yet. So the natural way to go is to go to higher energies. Though I don't know if that'll fly either, because there's the LHC and the supercollider were both based on the fact that there was reason to believe that things would happen in the TEV range. Once you see that things haven't happened in that range, I don't think there's a good theoretical argument to, other than, well, let's just look somewhere else. Look at higher energy. So--
So is that to say, Gary, just so I understand. If there isn't something that can be seen in the mid-range, jacking it up to double that doesn't, that suggests to you that we're not going to find anything there either?
No, I'm saying that the SSC and the LHC were motivated by theoretical ideas about where the natural place to find new physics, to find evidence for supersymmetry or whatever, so forth, was the TEV region. And it also had to do with limits on where the Higgs would be, and so forth. If the, what I'm saying is that if you don't see anything at that level, and you don't have any theoretical argument about why it should be at a slightly higher level, so you get double or triple level of the energy, then it's hard to convince people that they should spend billions of dollars on it. Now it may happen. The Chinese are quite interested. They've been very successful in pushing high energy physics. And doing it. So it may happen. But at the moment, we don't see a very cohesive movement in that direction.
So what you're saying is, let's say the Chinese do build this, right? And they don't find anything. Theoretically, that wouldn't be terribly surprising to you?
No, it wouldn't. I mean, there's... If it's not at the TEV level, then I don't think... Now maybe, maybe there'll be some theoretical arguments that come up, but I haven't seen them. That it should be higher, because one of the worries was, the grand unified scale is thought to be, for good reasons, thought to be many orders of magnitude above where we are with the LHC or SSC.
And the great worry that was expressed even when we were talking about building the SSC and the LHC was that maybe there's a desert, was the word being used, that the distance between the low energy physics and the grand unified physics has nothing in between. And there were arguments based on what's called "naturalness" that argued that there had to be something in between. But it pointed to a TEV level as opposed to a somewhat higher level. But these arguments were probably not that strong, so you could say that, well, you know, we should just explore further, but the problem is these things are very expensive. So it becomes somewhat discouraging.
Well Gary, I think for my last question, you know I want to ask, given your relative pessimism about what a new SSC would uncover, right? What are the areas of physics that you think are going to be most productive and most fruitful for the coming decades? In other words, if you found yourself really at the golden age of high energy particle physics in the 60s and the 70s and the 80s, right? What is that new golden age for the next three decades that you see?
Well, the one of the things that's really being pushed now is work on what's called quantum physics. The idea of using quantum entanglement to do interesting things like building quantum computers and other devices there. And that's a very active area in almost all of the physics departments. So that's one of the directions that seems to be very promising. The, I don't know as much about condensed matter physics, There's always, you know, it's a complex field, there's always room for new understandings and new discoveries in these areas. But again, it's not my area of expertise, so I don't know how likely all of that is. But those are certainly the energies that I see departments investing in.
The other area, actually that's become very attractive is actually atomic physics. And again in particular using some new techniques that have developed over the past couple decades. For instance, one of the things that people have done is to actually use, to build artificial lattices out of atoms controlled by lasers. So that you can sort of do condensed matter physics in a very controlled way. And this has become a very hot subject, also. Now, you know, something may be interesting in particle physics, but it would be certainly nice to see some hints from the LHC before we dive into new, multibillion dollar projects. Neutrino physics still has a way to go. And that's where the U.S. is putting most of its emphasis. As is Japan.
Who are some of your--
China might be going the other direction.
Who are some of your younger colleagues in neutrino physics who are doing some really interesting and exciting work?
Well, one of the interesting things that came out in the last few years in our experiment and it's branched out to everybody's experiment now, all sorts of experiments, is the advent of deep learning. It turns out that we, again, we had a couple-- So the traditional way of doing the kind of physics that we do is you write programs in order to basically take electronic pictures of events. Including measuring their energy and so forth. And then we write programs to reproduce that, to find the vertices, to find the tracks, to measure the energies. And then fit that to models of what we think the physics is and so forth.
What happened in recent years is that some of the young people caught on to advances in artificial intelligence. Now we have been using, let me say before I start that, we've been using artificial neural nets for a lot time to help make decisions, but these have been very shallow nets, and you have to feed them high-level information and you have to choose carefully what high level information you give them in order to make them powerful decision-makers. But people lapped onto this deep learning, which takes the opposite approach. And it was developed for two reasons. One is visual recognition problems, like facial recognition is an example. And the other was supposedly a natural languages. And the idea here is that you just give, the net is a much more complicated structure. In fact, it's based on learning how the eyes, particularly the eyes of animals, work. And it has a much more extended structure, and in our case you just give them the raw data.
In other words, you feed them the picture that we took. And they learn themselves, you train them by giving them examples of this kind of an event looks like this, and this kind of an event looks like that. And they train themselves to make these decisions. To wire themselves so that they can figure out classifications, the amount of energy that's there, all these. What each of the prongs is. All of these things. And we found, the first thing we used them for was just classifying what kind of an event they are. We have a number of kind of events, charge current events, neutral current events, charge current events with muons, with electrons, and so forth. And just used them for classification.
And they did much better than anything we had ever written. Right out of the box. It only took a couple months to show this. And now, it's become a real industry and people are using them for more and more things. So we used to have this reconstruction group that did this classical reconstruction, and now it's been renamed the Reconstruction and Deep Learning Group. So, it's been a real revolution in the way we do things. It gives us more power, but it doesn't change the physics that we're doing.
Well, Gary, it's been a delight speaking with you today. I want to thank you so much for spending the time with me.
Oh, I was happy to do it.