Notice: We are in the process of migrating Oral History Interview metadata to this new version of our website.
During this migration, the following fields associated with interviews may be incomplete: Institutions, Additional Persons, and Subjects. Our Browse Subjects feature is also affected by this migration.
Please contact [email protected] with any feedback.
This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.
Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event. Disclaimer: This transcript was scanned from a typescript, introducing occasional spelling errors. The original typescript is available.
In footnotes or endnotes please cite AIP interviews like this:
Interview of A. J. Stewart Smith by David Zierler on March 8, 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 A.J. Stewart Smith, the Class of 1909 Professor of Physics, emeritus, at Princeton University, who also served as the university vice president for the Princeton Plasma Physics Laboratory. Smith begins the interview with an overview of his affiliations with SNOLAB, CERN, and Italian Nuclear and Particle Physics. He recaps the effects of the pandemic on experimental particle physics. Smith then summarizes his family history and his childhood in Canada, where he became interested in the sciences in high school. Smith recalls his undergraduate studies in physics at University of British Columbia, where he also earned a master’s degree, as well as his decision to pursue a PhD at Princeton. He describes working on the Princeton-Penn Accelerator with his advisor Pierre Piroue, and the subsequent offer of a fellowship at DESY working with Sam Ting on QED. Smith recounts his move back to Princeton to join the faculty, and he describes the “bipartisanship” between experimentalists and theorists at the time. He discusses the origins of the Chicago-Princeton collaboration at Fermilab, his involvement with E-787 experiment at Brookhaven, and his time as technical coordinator and spokesperson for the BaBar experiment. The interview concludes with Smith’s recollections of his time as Princeton’s first dean of research, as well as his reflections on times when theory has led experimentation, and vice versa.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is March 8th, 2021. I’m so happy to be here with Professor Arthur John Stewart Smith.
Just call me Stew, please. Call me Stew. My parents felt that with a name like Smith I needed redundancy.
[laugh] It’s great to see you, Stew. How are you?
I’m doing fine. Still hanging in there but getting old. It’s hard to believe where all the time has gone.
It really is. And we’re at a year into COVID-19. We’re at a year.
It’s amazing. Stew to start, would you please tell me your title and institutional affiliation?
Sure. I’m officially the Class of 1909 Professor of Physics, emeritus, at Princeton University. This professorship was endowed by the class of 1909, and makes some people not able to resist saying, “Stew, I didn’t think you were that old.”
[laugh] Stew, how does that work at Princeton? The class of 1909, is there any particular connection to your work or was there a distinguished alum from that class?
For some reason that class decided, maybe they had a faculty member or a famous physicist in their class, to endow a professorship with that name. I would guess half of the tenured faculty in physics are endowed by something. Endowed professorships help the university to raise funds, by saying to prospective donors: “You too can have your name on —“
That’s right. Stew, when did you go emeritus?
2017. About four years ago.
[laugh] And in what ways of course have you remained active in physics and at Princeton?
Well, let me just say at that time I was the university vice president for the Princeton Plasma Physics Laboratory, which is one of the 10 US Department of Energy’s national laboratories for science. The lab is operated by Princeton University under a contract with the DOE, and I was the president’s delegate to deal with the lab and the Department of Energy. After retiring from that position I have continued to work at several research laboratories around the world. I jokingly say the only reason they want me is because they think I’ve made every possible mistake and might help prevent their repeating them.
In that respect I’ve been the chair of several scientific advisory committees, charged to evaluate the scientific potential and technical soundness of proposed experiments, and monitor their progress once approved. As one example, since 2007 I’ve led the Experimental Advisory Committee at SNOLAB, a Canadian laboratory 7,000-feet underground in Sudbury, Ontario. It’s most famous for an experiment done by Dr. Arthur McDonald and others to show that the neutrinos from the sun could oscillate among their three different “flavors” and that the solar neutrino flux really did obey the solar model. (McDonald received a well-deserved Nobel Prize for this work a few years ago.)
The laboratory is much larger now, operating and preparing a set of challenging experiments looking for dark matter, which is thought to make up ~20% of the matter in the universe, and neutrinoless double beta decay, observation of which would transform our understanding of the ever-mysterious neutrinos. I have had similar roles at CERN beginning in 2004, on the LHC committee, which oversees the experiments at the Large Hadron Collider in CERN. My assignment was to lead the oversight of the CMS experiment through design and construction, culminating in the Higgs discovery in 2012.
The CERN experiments soon required upgrades to deal with the increasing collision rates of the LHC, and in 2013 I was asked to a group that has to certify to all the 45 funding agencies at CERN that an experiment knows what it is doing when proposing an upgrade—and these things are expensive – with a total budget of 1.2 --1.3 billon dollars in US terms. The two major experiments, ATLAS and CMS, have each made about six upgrade proposals for parts of their detector, some exceeding 100 million dollars. (I don’t look at them if they’re less than 10 million dollars.) Our group gives them a “root canal” to make their estimates of cost and schedule are sound, that they have sufficient people, experts, and technical resources, and all the other things required to make the project likely to succeed. Typically, we met at CERN for a week four times a year. The big-time crunch came in 2017, and I’m glad I had retired from Princeton because we had 8 proposals to evaluate in one year, each of which involved close to a week of work and travel. Since then, the load has dropped to a couple a year, along with semi-annual follow-up reviews. I don’t know why they still want me, but I’m happy to keep helping CERN – it’s now the only “microscope” in the world that can probe for new physics beyond the amazingly successful but incomplete Standard Model. And it sure beats watching the grass grow!
For the past 7 years I’ve also greatly enjoyed helping Italian Nuclear and Particle Physics. Italy has a wonderful funding agency called INFN, Istituto Nazionale di Fisica Nucleare, which funds all Italian particle and nuclear physics and supports institutes at most major universities. INFN appoints an International Visiting Committee, a group of seven scientists from around the world, to meet once a year to evaluate their programs, and send a report to the Italian government. I’ve chaired this group since 2015 and will continue to do so for a couple of more years. INFN seems to appreciate our efforts, and I have found these meetings to be a great learning experience. It’s a bonus that we usually meet in very nice cities. Alas, last year we were supposed to meet in Venice but because of COVID we ended up meeting on ZOOM. And this year we’re hopefully going to meet in Venice, but I’m not holding my breath. [We did make it in 2021, and it was great!]
Seems we’ve got off to a start by discussing what has kept me busy since my own last major experiment, BaBar at SLAC, that finished in 2008. As I’ll explain later, I was dragged into administration in 2005, so these committees have been great, because it’s a way that I can keep at the heart of what’s going on while working full time as Dean. And having been on the “defendant” side of the table for decades, it’s been a refreshing change to be on the other side now and then.
[laugh] Stew, a very broad question, a very present question. Given the breadth of your collaborations, the fact that you have your pulse on what’s going on really all over the world, in the world of experimental particle physics, what is your overall sense of the extent to which the pandemic has really slowed things down? In other words, we can say that the theorists can always retreat into their world of calculations. Right? But the experimentalists cannot do that. What is your overall sense of what projects have really slowed down and to what extent is mask wearing and even using Zoom, doing remote data analysis really kept a whole enterprise going as well as it could have?
Very good question. First, since it’s the easiest one to answer, let me deal with the data analysis first. That has not suffered much. Well, except for the fact that there isn’t any new data! But there’s so much data around and the field has already moved incredibly well towards efficient remote participation. And I’m proud to claim that the BaBar experiment pioneered remote data analysis in 2001, with the first of what are now called “data grids,” the dominant model for research in particle physics ever since. With its high-bandwidth communication, BaBar scientists could work remotely from their institutes in North America and Europe. And you don’t have to know where the computations are happening, it’s all done transparently: the data may be stored at a computer center in one country and processed in another country. The BaBar grid is a great example of “necessity is the mother of invention.” We realized with dismay that we had completely underestimated the amount of computing needed. You just can’t get enough computing, as other experiments have also had to learn the hard way. The first attempt, to ask the participating countries to send money so that SLAC could build up more on-site computing, was a non-starter -- they said forget it! But then Guy Wormser, the French member of our International Finance Committee, later director of the Orsay Lab, said “look, we have a huge computer center in Grenoble that is dedicated for the Hadron Collider, but they won’t need it for several years. If you can figure out how to use it, it’s yours.” And so, that’s what led to the grid.
When you look back on it, it would have been a disaster to place all the computing at SLAC, because it simply wouldn’t have worked. Instead, there would have been a terrible log jam, especially for remote users, because communications were just not adequate in those days -- no data highway would be fast enough to handle all the traffic in and out of a single BaBar center. Serendipitously, the fact we had to disperse BaBar computing around the world turned out to be a huge advantage. This approach was taken up by CERN for LHC computing, leading to a fantastic, “Michelin 3-star” grid. It’s really the ultimate, and of course this model has become the model for other parts of science. That’s one of the things I like about high-energy physics, though we have to be extremely careful not to take credit where we weren’t the instigator. That said, distributed computing was a real triumph and is intrinsic to CERN experiments today.
After that digression, to answer your question. Most aspects of experimental particle physics the past year have been drastically affected by COVID, with the exception of data analysis, which because the grid allowed people to work from home, was not seriously affected. On the other hand, travel restrictions, laboratory closings, and lack of access to the experiments have been devastating. For SNOLAB it was extreme: beginning in early March 2020 there was no access allowed into the mine at Sudbury for six months. Nothing. And no travel was allowed, even within Ontario. Even now, a year later, it’s only in the last week that the province of Ontario has allowed people from outside the Sudbury area to come to the lab. All the experiments had to be put in a standby, safe-mode state -- hibernation would be a more accurate designation. SNOLAB is situated in one of the world’s largest operating nickel mines, and the mining company is terrified about safety. Just imagine: to reach SNOLAB, or to the active mining sites, you take a cage that impressively drops 7,000 feet in three minutes (four minutes for tourists so they don’t freak out, crammed in with 50 or so miners, all breathing the same air). With the pandemic, however, only five people are allowed per trip – down by an order of magnitude. Worse still, essentially all the places are reserved for the miners; access by SNOLAB people were limited to responses to emergencies or absolutely critical maintenance. Fortunately, restrictions for local people were relaxed in the early fall, to allow about one quarter of the normal number to come down, but those were only residents of Sudbury. To cope, a lot of the laboratory staff who were doing other things were trained on the spot, or via Zoom with the relevant expert -- “Do I push this button or that button?” This really slowed things down. I would say the average delay has been at least 9 months in a year. As mitigations, experiments are being designed and constructed elsewhere before being sent to SNOLAB, and some of the components were already underground. Unfortunately, the border between U.S. and Canada is still completely blocked, seriously impacting one of the most exciting possible new experiments at SNOLAB, a dark matter search involving cryogenic solid-state detectors. The collaboration, called SuperCDMS, is largely US based, so they can’t do very much because most of the work requires more expertise than can safely be transferred to locals.
So much for SNOLAB. At CERN, again there was really almost nothing for six or seven months at least, and similarly to SNOLAB, until recently only a small group has been allowed into the lab, but with a few exceptions, they are at last making reasonable progress. The schedules for running the High-luminosity LHC and producing the detector upgrades have been delayed at least a year, and we all think an additional delay of six months or a year is likely. The delays are not only caused by COVID, however, because the upgrades are so far beyond the state-of-the-art that unexpected problems crop up that take time to solve. The plan is to run for 10 years, but it will be 7 years or so before the LHC can operate at the final collision rate (6 or 7 times higher than now) with the final upgraded detectors. These delays seriously impact the continued availability of expertise and damage careers, especially for younger scientists, especially at universities where the tenure clock is always ticking.
Well, Stew maybe let’s talk about some happier times. Let’s go all the way back to the beginning in Canada. Let’s start first with your parents. Tell me a little bit about them and where they’re from.
My parents were from the U.K. My father was a Scot from Edinburgh and my mother was born in London. Her family immigrated when she was only about 6 years old. They were told that there were great opportunities on the plains of Canada, and left for Regina, Saskatchewan. My mother didn’t like it there at all, and as soon as she could headed west in the late 1920’s for Vancouver where she worked as a legal secretary. My father was a bit of a character. He went to the Royal High School in Edinburgh and then worked in a bank in London. His boss soon offered him, and he accepted, an opportunity to see the world and work in the bank’s office in Madras, India. He took a ship all the way through the Suez Canal to Aden, where he was to transfer to a ship bound for Madras. It was a constant 120 degrees while he had to wait in Aden for more than a week, and as a kid from the north he was allergic to the heat. Madras, one of the hottest areas in India, was little if any improvement, and the colonial culture was rife with racism, brutality and excess, as chronicled in movies like Gandhi or Jewel in the Crown. For example, when dad arrived his host told him: “Mr. Smith here are your six servants.” He’s a kid; 18- 19-years old. He thought, “What do I do with a servant?”
Almost immediately he became ill with typhoid fever, which kept him in bed for a couple of weeks; between the sickness, heat, and disgust with what was going on he decided to return to London. He attributes this decision mostly to his outrage at the way the British were treating the Indians, but the terrible heat was certainly a major factor – in any case, only 6 weeks after arriving he just got back on the boat to England and told his boss, who had paid for his trip to Madras and back: “Sir, you won’t believe what they’re doing to those poor people in India. It’s terrible. Let me tell you about it.” The boss replied, “No need sir. Mr. Smith, your career in Britain is finished.”
After thinking things over, he sailed to Montréal, which he liked, but his lack of French made him feel uncomfortable and he kept heading west. By crazy coincidence, he actually stopped and worked in Regina for a few months. Though he didn’t meet my mother he got to know and like her brother. However, like my mother he couldn’t stand Regina and took the train to Vancouver BC where, realising the next step would be Japan, he was stuck. He met my mother in 1929 just before the depression hit, and after years of saving money together they married in 1936. I wonder what took them 7 years!
Dad worked for the BC provincial government, first in Vancouver and later in the capital Victoria as head of the equivalent of the Securities and Exchange Commission. It was a challenging job at that time because BC was a prime source of phony gold stocks, and his job was to stop the bad guys. He really liked this work, and indeed it was pretty interesting. When he retired, he thought he would try “the other side” as a financial advisor. However, he felt it would be uncomfortable working in B.C. after having policed his would-be future colleagues, and was convinced by a friend to go to Australia, where he spent his last 10 years in Sydney and in Perth.
A real child of the British Empire; India, Canada, Australia.
They were really good parents. They did everything for me and my sister, and I guess the most important thing for me was their moving from Vancouver to Victoria when I was 8. Victoria was a very strange place in those days --“more English than the English.” Originally established in 1843 as a Hudson’s Bay Company trading post, Victoria was transformed in 1846 when the United States took over the Oregon territory from the British and had serious thoughts about continuing north. “54-40 or fight; etc. --why not keep going right up to Alaska and grab what’s now British Columbia?” Naturally the Brits didn’t like that kind of talk so they established their main Pacific naval base in Victoria. It’s still there, but now of course as Canada’s main base.
Britain had learned from its mistake in the Oregon Territory and wouldn’t let the U.S. settlers in covered wagons outnumber the Brits. Emigration from Britain to B.C. was strongly encouraged -- an easy sell because there were jobs: Vancouver Island was the biggest coal supplier for the whole west coast, and the British fleet contributed strongly to the economy. Fortunes were made, and an awful lot of wealthy English arrived. The joke was that a good fraction were “remittance men:” “Son, you have to leave immediately for Canada because we can’t have you disgracing the family any further. You’ll be well taken care of financially, but you must never set foot in Britain again.” And so, these guys came to Victoria. They literally built castles which you can see to this day. I grew up just at the very end of that period, when children or grandchildren of these characters were still rampant: English open roadster cars with steering wheels on the wrong side, handlebar moustaches and hunting hats – one could be excused for thinking Sherlock Holmes was driving down the street. My father being Scottish, and especially after his experience with the London bankers, didn’t have much love for this kind of Englishman. Fortunately, we got to meet loads of wonderful English immigrants after WWII to go along with the Scots, who of course could do no wrong!
[laugh] Stew, what kind of schools did you go to growing up?
Victoria’s schools were outstanding, but in part for the wrong reasons. The crazy cultural norms prohibited women working in many areas, with the result school teaching was one of the best jobs an educated woman could get. And the way it worked was really cruel: there was an unwritten understanding that women would quit their jobs if they got married so that single women could live. As a result, at least half of the teachers were unmarried women, and the school was their life. They were good, incredible, well-educated! It was the best job they could get. They put their hearts and souls into it, and thanks to them I would rate my education with that of any prep school. Victoria High School in particular was a truly special place. Founded in 1876 to teach the landed gentry, it is the oldest high school in Canada west of Toronto, and the oldest in North America northwest of the line from Minneapolis to San Francisco.
My high-school class has remained close ever since graduation, holding reunion lunches every year and a gala event every 5 years. A few years ago, after one of the reunions I went to the school to offer a gift, making sure the government could not “confiscate it” by reducing their support of the school. I asked the Principal to “Give me an exciting project you would do if you had, say, 10 or 20 thousand dollars.” He quickly came back with a tremendous idea. They had recently hired a physics teacher who had just finished his PhD in astronomy and chosen to pursue a career in high school teaching instead of research. He said he would love to start an astronomy course but couldn’t get money for the telescopes, computers etc. to make it viable. This was music to my ears. With my donation the course is oversubscribed and widely known in Canada, and the local community enjoys star parties on the roof. Fortuitously, the school building had to be extensively remodeled to meet modern earthquake-safety regulations, and we’re all delighted that the budget includes a couple of million bucks for a new rooftop designed for astronomy and star parties. I feel very good about this and am now working with the school to find another irresistible project to fund.
Stew, when did you start to get interested in science?
In high school, most likely because after the war everybody was talking about atom bombs, radar and things -- “Wow, how did they do that?” We also had an extremely good chemistry teacher who started every class with a question, and then answered it via dramatic explosions, psychedelic color changes, and so on. He should have been a performer.
Do you remember his name, the chemistry teacher?
Yes indeed. Douglas Wallis, affectionately nicknamed “Test-tubes Wallis.” In those days the teachers, like at prep schools, were also the coaches of the athletic teams. “Test-tubes” was the junior rugby coach, and I guess I respected him at least as much as coach as teacher. We really got along well, and I would often go in and ask questions.
The two biggest schools in Victoria competed “viciously” to have the best students in the BC-wide college entrance examinations, similar to the NY Regents Examinations, and to claim the top student in the province. To this end, in September of grade 12 Vic High selected about 15 or 20 of us to be so-called “scholarship students,” and brought us in at lunch and after school for extra work. This paid off, as year after year, though only 5 percent of the BC student body regularly beat our Victoria competitor and all the Vancouver schools. I was very proud to be the 1955 winner and remain increasingly appreciative of the teachers for their dedication and support.
I initially decided to be a physics major because I thought it was the easiest subject. More interested in sports than academics at that time, even though I really liked English and History I quickly tired of writing term papers -- it was too much work. And then Sputnik changed everything and made me so happy I was well along the way in physics.
Stew, did you know that it was going to be experimental physics, even in undergraduate? Or were you open when you were thinking about graduate programs?
I loved doing experiments. And of course, was lucky I didn’t hurt myself mixing chemicals for Halloween fireworks.
Who were some of the professors in college that you became close with or who exerted a formative intellectual influence on you?
I would single out two UBC professors, both immigrants to Canada after WWII. A Polish theorist who taught us quantum mechanics, thermal physics and classical mechanics, and an English nuclear physicist who had come from Britain to build a Van de Graaff generator in the basement of the physics building, and who was also an outstanding, though arrogant teacher. My degree program, “Honors Mathematics and Physics” required lots of math courses and most of the math professors were quite good, but they constantly put us down with comments like “You’re good, but you couldn’t lick the boots of the student who was here two years ago.” When I later found out they were comparing us with Robert Langlands, soon to be one of the most famous mathematicians in the world, I didn’t feel so insulted though it would have been nice if they’d been more diplomatic.
Langlands left a legend that lasted for a long time. He grew up in what was then a small village 30 miles from Vancouver, and wasn’t planning to go to university until his high school teacher insisted, and the rest is history. Amusingly, he has been a neighbor in Princeton for many years, and it has been fun to compare our professors.
Stew, what were some of the experiments or laboratory work that you did as an undergraduate that were really important for your intellectual development as a physicist?
I liked most of the labs, especially in electromagnetism where Maxwell’s equations come to life, and quantum physics. Education was slower then: calculus wasn’t taught in high schools and we didn’t start till halfway through our freshman year. By contrast the experiments were quite interesting, emphasizing physical optics and “modern physics” via a lot of famous experiments, most of them only 20-40 years from discovery, including Franck-Hertz, radioactive decay, Zeeman effect, atomic spectroscopy, photoelectric effect, the oil drop experiment, speed of light. The jumps and changes in currents, brightness, colors as we varied voltages, pressures, etc. fascinated me.
Stew, what kind of advice did you get about the best graduate program to apply to?
Not much. That was really crazy, especially the first advice, because everybody in Canada was expected to do a master’s degree to qualify for a PhD in physics. It’s optional now, thank God, but it wasn’t then. I should’ve left for the U.S. at that point, and it was dumb not to. I took a master’s degree with the professor on the Van de Graaff, but frankly the physics was old fashioned and boring because the machine was over the hill. As a result, I focused on playing top-level rugby and lacrosse.
Fortunately the UBC nuclear-physics graduate course emphasized new discoveries in what is now called high-energy particle physics: all the new particles being observed in cosmic rays and at accelerators, parity violation, two-component neutrinos, etc. This was fantastic! “Boy, that’s what I want to do – forget nuclei.”
As to where, I don’t think my professors had tremendous knowledge. When I asked where to go for PhD studies they just said, “McGill or Toronto, or stay with us.” As to US universities, I should try Harvard, MIT, Wisconsin and Caltech. Only one professor, a theorist, mentioned Princeton. Part of the problem was they were mainly Canadians or British who hadn’t traveled much in the US. They also didn’t realize how early the deadlines were in the U.S., so when I wrote for applications Caltech and Harvard simply said “sorry you’re too late, apply next year;” Wisconsin said “great, take your time;” and fortunately MIT and Princeton were very, very supportive. They said “Okay, please send your stuff as fast as you can and we’ll try to work you in.” And they both did, offering nice research assistantships. It was a terribly painful decision -- I didn’t know much about either of them, and only got superficial advice, along the lines of “go to MIT, they’ve got the equipment.” Actually, both MIT and Princeton were building new accelerators: the Cambridge Electron Accelerator (CEA) at Harvard/ MIT, and the Princeton-Penn Accelerator (PPA) in Princeton. I know now Princeton was the right choice for me, but, on the other hand, I’ve spent a lot of time working with MIT people and it would also have been wonderful. MIT has put me on their advisory committees many times. I just love the place and the people who work there.
Stew, when you got to Princeton, how did you go about identifying a graduate advisor for you?
The first two years were well-defined: prepare for a wide-ranging qualifying examination, and work for an experimental group supported by a research assistantship. Courses were offered but not required –I didn’t have to repeat courses I had taken at UBC, perhaps the only benefit of the master’s degree. I wanted particle physics and was delighted to be assigned to the Elementary Particles Laboratory (EPL), located in a little temporary building behind the stadium built for John Wheeler as a shock wave laboratory for the Manhattan Project. The most exciting thing was that all the people – professors, postdocs, students, and technical staff – ‘lived” in the EPL, all the time, day and night, and we had a lot of fun together in our own little world: picnics, informal talks, Christmas parties, and in summer the daily 5:30PM volleyball games. Frequent faculty participants included Jim Cronin, Val Fitch, Sam Treiman, Pierre Piroué, Dave Hutchinson, Dave Bartlett and Dino Goulianos among others. Already eminent, Val was a member of the president’s science advisory committee (PSAC) and brought back news from Washington. As a draftee in WWII Val was assigned to the group on the Manhattan project who built the trigger for the Trinity test and emerged an expert on “fast electronics.” He even built a sophisticated oscilloscope while he was there that anticipated the famous Tektronix scopes.
It soon became time for thesis research. The lab director gave me great advice: work with Pierre Piroué, he’s a good young guy who’s going places. A Swiss chap who had just become an assistant professor, Pierre was looking for his first student to work with him on some of the initial experiments at the PPA. Though the new Brookhaven AGS might have been more exciting it was great to work with Pierre, and the PPA was only 2 miles away. Pierre and I were the only two people on our experiments, and I liked being left alone to work out problems myself.
Stew, what was Pierre’s research? What was he working on when you connected with him?
The PPA was brand new when Pierre and I came on the scene. The first thing to do was to characterize the particles it produced, and make a survey of particle production at various angles and energies – what is this accelerator producing? Pierre had designed the equipment for what was called the beam survey and led the team carrying it out. We would move our detectors to make measurements of particle spectra at the several different angles where secondary beams would be built for future experiments. This phase was really bread and butter, but perfect for me to understand how to make proper measurements. Our work became much more exciting when we got a visit one day from Leon Lederman, who had recently completed the famous “two-neutrino experiment,” and a young Columbia assistant professor Sam Ting. One of the experiments Leon, Sam, and Wonyong Lee were doing was to search for anti-matter at the Brookhaven AGS. Anti-protons were old hat by this time, but anti-deuterons, anti-tritons, etc. would be new. However, the cross sections for producing such massive objects crashes as you go to low collision energies, even at the AGS, the highest-energy accelerator at the time. Fortunately, the (Fermi) motion of protons and neutrons bound within a target nucleus could effectively serve as a “poor-man’s” colliding beam, significantly increasing the energy available in individual collisions. This means, because some of the nucleons will be moving towards the projectile, increasing the effective center of mass energy, in some cases rather drastically, particles can be produced with masses greater than energetically possible for a beam proton hitting a free proton in the target.
It was therefore important to measure the Fermi-momentum spectrum for heavy nuclei by seeing how low we could go in proton beam energy and still produce anti-protons. Antiprotons are produced as part of a proton-antiproton pair, requiring a center of mass energy greater than 2 proton masses (1.9 GeV), or laboratory proton-beam energy greater than 5 proton masses (4.7 GeV). To be well above this threshold, the Berkeley Bevatron, built to discover the antiproton in 1955, was a 6-GeV machine. Leon’s idea was to search for and measure antiproton production below threshold, even lower than the Bevatron could provide, and thus determine the Fermi momentum distribution. “Let’s go as low as we can.” The PPA energy was perfect –well below threshold at 3 GeV – and the amount of antiproton production would measure the high-momentum tail of the Fermi distribution. Indeed, Pierre and I found some events, and had the lowest energy points on the cross-section curve of antiproton production. As part of my thesis I used this data to extract the Fermi-momentum spectrum, which Leon et al. used to calculate their sensitivities and cross-sections for producing anti-matter at the AGS.
As I was finishing my thesis in the fall of 1965, what to do next –where to go? As opposed to 4 years before in Vancouver, this time I was in the perfect place to find a new job. The famous Princeton discovery of CP violation in 1964 and the 1965 discovery of the cosmic microwave background radiation 30 miles away in Holmdel transformed Princeton all of a sudden into a mecca for the physics that turned me on. Cronin had just returned from a sabbatical in Paris with René Turlay, his former postdoc on the CP experiment. Jim felt terrible about leaving Princeton at the height of the CP excitement, but felt he had to honor his commitment. When he returned in September 1965, he agreed to be second reader on my thesis, at a time when I was figuring out what to do. He knew I wanted to go to Europe and said he was sure René would like to have me work with him at the Saclay laboratory near Paris, and perhaps also at CERN. I was all set to go, when Sam Ting called. As I recall it was December, the day I passed my final PhD oral. Sam said, “Stew, how would you like to go to Germany for an exciting experiment, how would you like a fellowship? Let me come down to Princeton and tell you about it.”
While writing my dissertation a few months before this I had met Norma, a smart and beautiful Norwegian-American girl, on a blind date at an NHL hockey game at Madison Square Garden. She came to Princeton the day of my oral exam, hoping to celebrate my passing the oral. The examination committee was rather daunting: John Wheeler, Murph Goldberger Val Fitch and Jim Cronin. After the oral, which somehow I passed, Cronin invited me to his house for a drink, and I asked Jim if my girlfriend could join me. When I saw him the next day, he gave me the first of his several life-defining pieces of advice: “Stew, I hope you aren’t dumb enough to let her go!” I passed this test with flying colours --more later.
Sam came down a few days later to describe a huge kerfuffle in the field at this time, triggered by an experiment at the Harvard-MIT Cambridge Electron Accelerator (CEA) that claimed quantum electrodynamics was violated in the production of electron-positron pairs. Many groups wanted to confirm this earthshaking result, but the only places in the US available were closed to “outsiders:” An older 3-GeV machine at Cornell, or the Harvard-MIT CEA. This spurred Sam to fly over to Hamburg, where a new accelerator, the 6-GeV Deutsches Elektronen-Synchrotron (DESY), had just turned on. DESY needed all the help they could get, partially because so many of their young scientists had been wiped out by the war in one way or another. They just didn’t have senior scientists, so I guess they were hard up enough to let this 28-year-old young guy Sam Ting come in.
Sam’s offer excited me greatly, and I was inclined to take it. Before doing so, however, I went to see what Cronin thought. As usual he was quick and to the point: “if you want an enjoyable stay in Europe, go to Paris with Turlay; if you want a career in physics, go to Hamburg.” I did.
The Hamburg group consisted of eight people: On the US side were Sam, three postdocs and a student; on the German side one 38-year old scientist and two students. That was it – 8 people. The German scientist, a very nice person, was not really up to leading an experiment and struggling, so Sam quickly took over. The two German graduate students were really good, especially Ulrich Becker, who went on to have a fabulous career at MIT and make critical contributions to all of Sam’s future experiments at Brookhaven and CERN. The two US postdocs other than I, Joseph Asbury and Bill Bertram, had already been at DESY for a few months working on another experiment that wasn’t exciting, and quickly transferred to Sam’s group just before I arrived in March 1966. In the meantime, I remained in Princeton writing Phys. Rev. papers on my thesis work and building some equipment to bring to DESY. When I arrived at the German customs in Frankfurt, they were not going to let me bring in the equipment. When I asked why, they said it was because they couldn’t understand what it was. The officer asked me: “Why are you coming to Germany?” I said, “I’m going to Hamburg to work at DESY and have brought this equipment for my experiment there.” “Oh,” he said, “Do you have any proof?” I gave him my offer letter, which described my job and of course also stated my salary, which was at least 2 or 3 times higher than German salaries, to be competitive with US salaries. The customs officer then smiled and said “Was für schönes Geld! Willkommen in Deutschland [What beautiful money! Welcome to Germany]” and allowed me in and my stuff to enter.
Norma and I had decided to marry in Europe – originally in Paris, and then Hamburg after I chose the DESY offer. She came over on the US United States with all our possessions a few weeks after I arrived, and I met her at the dock in Bremerhaven. We were married at the Hamburg-Altona Registry Office on May 20, 1966, with her Norwegian cousin and husband as witnesses. I don’t know how she tolerated being essentially alone in a new country with my working day and night, but we made it through till the experiment ended in August, when we took a month off to explore Europe in our new Alfa Romeo sports car – our deferred honeymoon. The fact it was partly a “busman’s holiday” – I presented our results at a summer school in Herceg-Novi, Montenegro (not far from the Albanian border) – actually enhanced the experience as we made several lifelong friends, and the journey through Switzerland, Italy, and down a newly opened road along the Adriatic coast gave us a truly special introduction to Europe. We even were delayed for an hour by President Tito’s motorcade!
The central goal of our experiment was to repeat the Harvard experiment and either confirm or refute their result in time for the biennial International Conference on High Energy Physics to be held at UC Berkeley in August (1966) – only 5 months away. I took the responsibility to study the PhD thesis from the Harvard experiment and found many things that just didn’t make sense and made me confident their result was not credible. It was somewhat sad, because the leader, Frank Pipkin was a highly-respected physicist and outstanding human being. I’m not sure, but suspect he let his students run the experiment without getting sufficiently involved himself. In any case we were sure the experiment was suspect, even though another experiment had been done at Cornell in the meantime and claimed to confirm the Harvard result. However, we were not impressed -- this experiment simply did not have sufficient sensitivity to make a conclusion one way or the other. Unfortunately for them they guessed wrong.
To make a long story short, everything in our experiment worked like clockwork. The DESY staff and senior management gave us outstanding support, and we all worked our asses off. The experiment was beautiful --it was a classic -- and our small team had all the needed expertise, and everyone took on major responsibilities. Looking back, it seems amazing because there were only eight of us and it was a big, difficult experiment.
What was beautiful about it, Stew?
First, the detector, a so-called two-arm pair spectrometer, was extremely well designed and built –everything worked right off the bat. We didn’t have any problems with equipment breaking or not meeting specifications, and relieving one of our biggest worries, the dreaded background levels turned out to be just as predicted. Unlike today, only primitive methods were available to simulate and estimate the mess of x rays, γ rays, electrons and neutrons that an intense photon beam makes when it strikes a target or eludes beam-defining collimators, so we were either conservative or fortunate, or both. All these backgrounds are neutral and cannot be deflected out of harm’s way by magnets. Worse, they produce showers of low energy particles when they encounter material of any kind. To get rid of this nasty stuff we designed a set of collimators and magnets to block them, sweep out low-energy charged particles, and allow a clean well-defined path for the e+ e- pairs produced in the target. Miraculously the data acquisition, analysis software, and efficiency calculations were ready on time, so we could analyze the data almost in real time and complete the experiment in time for the conference. Our results overwhelmingly confirmed QED and nobody challenged them, not even our competitors, putting us on the world scene. What a rewarding introduction to competitive experimental physics!
This was only made possible by the outstanding leadership of Sam Ting. Even though it was his first independent experiment, he was in command of every facet of the experiment down to the smallest detail and kept coming up with ideas for improvements. My favourite example came on a Friday afternoon a few weeks into data taking, when he asked me to find a way to build a Cerenkov counter to fill a 4-meter-long stretch of the spectrometer arms, to increase our ability to select electrons in the presence of a much larger pion background. This initially seemed insane, as the experiment was completely built and running, and there would be no way to insert traditional devices without dismantling the experiment. Graduate student Charles Jordan and I came up with a solution: plywood boxes -- one for each arm – supported on shielding blocks. We painted the insides black, sealed the joints with silicon cement, and for a reflecting lens, we bent a piece of aluminized plastic to focus the Cerenkov light onto a set of spare phototubes that were lying around. For the radiator we simply flowed freon through the boxes (today this would be completely forbidden, but not at the time). Charlie and I built these counters, affectionately known as “The Coffins,” and had them operating in just one weekend, gaining a factor ~100 in pion rejection. And so it went!
Our group got our 15 minutes of fame for confirming QED (people thanked us, but often quipped something like “I always knew QED was good”) and this put Sam Ting on the map. More important, as with many experiments, something we weren’t looking for turned out to be more interesting. Whereas the invariant-mass spectrum of e+e- pairs agreed with QED up to pair invariant masses of 500 MeV, an excess began to appear for higher masses, taking on the shape of a resonance peak at 750 MeV, the mass of the recently-discovered ρ meson! One of a family of vector mesons (ρ0,ω0, φ0) the ρ0 would be described in the then 3-year old “quark model” of hadrons as a quark-antiquark (q- q) pair, and its decay would proceed via q- q annihilation. Vector mesons have the same quantum numbers as the photon, and hence neutral ones could also decay by annihilating directly into a virtual photon, which could decay into an e+e- pair. And so we studied these pairs over the next year, testing QED up to ~500 -600 MeV pair mass, and searching at higher masses for leptonic decays of ρ0, ω0, φ0. We improved the photon beam and found beautiful signals ρ0,ω0, and later φ0 decays as well, into electron-positron pairs, providing solid confirmation to what was called the vector-dominance model. This was really very good stuff and opened the door to a new physics dimension for the e+e- storage rings under construction at Frascati, and later at Stanford.
As an aside, a few months after the Berkeley conference Frank Pipkin came to Hamburg to see our experiment. We were all impressed by his intelligence and graciousness, and I learned what I should have known, that he had been a Princeton graduate student of Donald Hamilton, working in atomic beams. Accordingly, I was the person who showed him the detector, and took him to the infirmary when he hit his head on the sharp edge of an overhead cable tray. He jokingly promised to assure people that we hadn’t assaulted him.
By this time, I wanted a faculty position and had offers from MIT, where Sam had moved from Columbia, and from Princeton. In August 1967 I decided on Princeton. Even though I loved working with Sam the fact I would be based in Germany with teaching duties in Boston seemed too difficult for my family, as we now had a new baby, Peter, born in Norway on the 4th of July. It was a good move because I think I had learned everything I possibly could from Sam, and staying on would have just been working for him.
He’s a hard worker and he expects the same of everyone else.
Indeed, and as long as I was there, I completely supported this. But things began to change as the experiment entered a more mature “precision phase,” in which new discoveries were less likely. This greatly concerned Sam as he tried to keep the program as relevant as it had been, resulting in increasing stress on Sam, and on the group. The work on vector meson decays continued through 1968, exploiting the fact that one could measure nuclei radii by coherent production of electron pairs. Compared to the first two years this was bread and butter physics, and then thunder struck! The discovery of deep inelastic scattering at SLAC in 1968 revealed a whole new substructure of nucleons—quarks and gluons. Nobody cared about nuclear-radii measurements anymore --they just didn’t -- and I guess Sam attributed the problem to the fact that he wasn’t working hard enough. In response he made everyone work even more intensively.
Stew, I wonder if you can explain a little bit more about the science behind how the experiment so overwhelmingly confirmed QED. In other words, even if the theory is solid, what was it about the experiment that was so compelling?
At this time QED was the only “complete” theory in particle physics. Among its precise predictions was the rate for producing positron electron pairs in photon-nuclear collisions, which would decrease in a predictable and dramatic way as the mass of the pair became a more significant fraction of the photon energy. So, in other words, it was much harder to produce high-mass pairs, as you might imagine. The quantitative predictions of it were very clear, right from Feynman’s rules and all that, making it a great situation to probe this beautiful theory – push on it experimentally to see if it breaks at some point – if it does, the form of any violation will point the way to a new, better understanding of nature. The Harvard experiment had found a large excess of these pairs, generating great excitement and begging for an experiment to confirm or refute their result. In fact Pipkin gave a colloquium at Princeton just as I was deciding what to do, which strongly confirmed my decision – off to Hamburg.
Stew, when you accepted the offer at Princeton, what was your sense of your prospects for tenure at Princeton?
Looking back, the situation at Princeton in those days is hard to believe. The department consisted of about 25 tenured faculty, and close to 40 non-tenured faculty: 20 assistant professors and 20 instructors. Imagine: 40 junior faculty and probably one tenured opening a year, leading to a promotion probability the order of 10 percent. Junior faculty were half-supported on research grants; an instructor was essentially a postdoc who did teaching and hired with that in mind. The business model of Princeton, Harvard, MIT and similar top research universities was very simple. (Harvard assistant professors were supported ¾ time by grants!!) Basically, the sales pitch went something like this: Come to Princeton! You’ll have great students, financial resources, technical support, light teaching load, etc. You’ll stay with us for a few years and then go on to a better job than you could get by going directly to a larger school. This “up-or-out -- mainly out” business mode worked because in the ‘60s there were loads of openings at really good schools like Michigan, Wisconsin, UCLA. Furthermore, transitions were simpler because almost all families were still single-career: the husband slaving to get a tenured job while his wife was at home taking care of the kids.
Perhaps a blessing in disguise, the department made the tenure decision quickly – in the fall of only the third year. In a typical year, three or four assistant professors in the department came up for tenure (others had seen the writing on the wall and found a new job). Thus the “standard” assistant professor trajectory was a three-year appointment, plus a terminal year to find a new job. That’s what I expected, so during my second year I began looking for places to go.
I went to Vancouver to see if UBC had any interest in me, as I had no particular desire to go anywhere else in the U.S. and would have been very happy to go back to Vancouver. UBC had greatly improved, with an exciting accelerator was about to turn on across the campus at the new TRIUMF national laboratory, and Vancouver is a wonderful place. If I could relocate Princeton University to Vancouver, that would be the ultimate! This visit accomplished my goal – I received an offer – so now we would at least be able to eat.
Back in Princeton, the dreaded November 15 date approached. As was the tradition I knew a letter from the chair, Murph Goldberger, would appear in my mailbox late that afternoon, announcing my fate. cold turkey: no meeting with the chair, no human contact. (When I later became chair, I made absolutely sure to deliver such news in person). I opened my letter, to be shocked at the unprecedented decision: a second three- year appointment. Clearly, the department had finally realised three years was not long enough. Murph went on to say, and I’ll never forget his wording “Stew, this is a big step, but in these days of fiscal peril nothing is guaranteed.” The others in my cohort were not so fortunate, including a brilliant young theorist -- John Schwartz of string theory fame! Tenure in theory was even harder to get because there were only 9 senior positions -- a “murderer’s row at that time: Wheeler, Goldberger, Treiman, Wightman, Bargmann, Simon, Hopfield, Callan and Gross.
A year later, when I was all prepared to go to Vancouver, my guardian angel Jim Cronin decided to move to the University of Chicago, creating an opening right when I needed it. In quantum mechanics the probability of a process is the product of matrix element and phase space. Similarly for the probability of getting a tenured appointment at Princeton: a person with an infinite matrix element would need only negligible phase space, but most people would need a finite amount, because there wasn’t much chance of expansion. I guess my matrix element was adequate, but without Cronin’s phase space I’d have been out of luck.
I was only one year into the second appointment when the good news arrived, via my former thesis advisor and then next-door neighbour in faculty housing. I was walking to my car when he walked up and said, “I think you should know the department has decided to recommend your promotion.” I was simply astounded by this unexpected development, which was followed by three- or four months of limbo as the University went out for letters before approving the departmental recommendation. I was passing by the department office one day when Murph waved to me with a big smile and said, “The president has approved.”
Stew, both as a graduate student and as a junior faculty member, what was your sense of the hierarchy in Princeton physics between experimentalists and theorists?
There was an unbelievably positive “bipartisanship.” Not like today in Washington.
The particle theorists, led by Sam Treiman and people at the Institute for Advanced Study (notably Abraham Pais, Steve Adler, Roger Dashen, C.N.Yang and T.D. Lee), interacted closely with the experimental community. Sam had come from the University of Chicago to Princeton as an experimentalist, but soon switched to theory, devising clever experimentally testable predictions, and guiding experimentalists toward the most important experiments to do. A most friendly and self-effacing person, he attracted the best students for many years. For example, his first student was Steve Weinberg, about whom he said, “my only job was to stay out of Steve’s way.” A couple of years later Sam attracted two star students in my class, Stephen Adler and Curtis Callan. Experimental particle-physics seminars attracted 50-60 people, a wonderful mix of experimentalists and theorists from the department, experimentalists from the Princeton-Penn Accelerator and theorists from the Institute. When discussing possible experiments with Sam I fondly remember his highest compliment: “You know Smith, that might not be dumb.”
My graduate class was very strong in theory. In addition to Adler and Callan, we had Alfred Goldhaber, son of Maurice, and Arthur Jaffe, who became a professor at Harvard. This group hung out in the attic of Palmer Lab, and unlike the faculty were somewhat remote from us mere-mortal experimentalists. The next year’s class, with Kip Thorne, Fred Gilman, Jon Rosner and George Bertch, was also no slouch, to say the least. Perhaps following Sam Treiman’s example, the theory students were all most friendly and helpful to us dumb experimentalists.
More broadly, the experimental faculty were extremely good and could hold their own: Bob Dicke was the star, with an outstanding group around him including Jim Peebles and Dave Wilkinson (the W in the famous WMAP satellite experiment stands for Wilkinson). Experiment and theory came even closer together with the Princeton discovery of CP violation in 1964, followed by Cosmic Microwave Background radiation a year later at Bell Labs, 25 miles away. A most exciting time and place to be a student!
Gerry O’Neill, a most unusual scientist, was also there. Gerry was a futurist; he really was, and as it turned out, a somewhat tragic figure. Gerry came to Princeton to be Associate Director of the then-forming Princeton-Penn Accelerator Laboratory. Almost immediately, however, he turned his attention to an exciting new idea – colliding beams – and with Burt Richter built the first electron-electron collider at Stanford. They did several beautiful experiments to test and confirm QED, but because the results agreed with QED their impact was less than earthshaking.
When you say it was not earthshaking. Why not? What was the issue?
As Burt Richter said in a 1982 review of Collider Physics: “These were important results, but the main contribution of the Princeton-Stanford storage rings came in accelerator physics and not in particle physics. Had QED failed, of course, the main contribution would have been remembered differently.”
At this time everyone was trying to understand the structure of hadrons in terms of quarks, and the SLAC results in 1968 left no room for others on center stage until the early ‘70’s when Burt and his SLAC colleagues began building SPEAR, a 6-GeV electron-positron collider (by this time Burt and Gerry did not get along and Gerry was excluded), with much greater physics potential. Gerry was now working on colliding beams at CERN and is credited with inventing the first hadron-hadron collider, the 60 GeV 300-meter diameter CERN ISR (Intersecting Storage Rings). This finally put CERN on the map – previously it had a 30-GeV proton accelerator similar to the Brookhaven Alternating-Gradient Synchrotron (AGS) but the transformational discoveries were all at the AGS (2-neutrinos, Omega minus, CP violation). The ISR, followed by the SPS collider put CERN at the energy frontier into the 1980’s when it was surpassed by the Fermilab TeVatron. Once again Gerry was the genius who got the whole thing started, but who again was marginalized because he was difficult to work with and confined to doing an uninteresting experiment. He asked me to join him, but I decided no. Other prescient O’Neill ideas: the wireless office, and an e+e- collider tuned to produce φ mesons, which decayed almost entirely into a correlated 1-K0K0 state, opening a new theater for CP violation. This lay dormant for decades, to emerge 30 years later as the DAFNE e+e- collider at the Frascati lab in Italy, which has tried valiantly to advance the field with mixed success. Had it been built when Gerry invented it, things could have turned out much more positively.
Gerry had wanted to be an astronaut and was one of the finalists but didn’t quite make it. In 1969 it was his turn to teach the main freshman physics course; fascinated by the Apollo landing and space physics in general, he used space-physics examples at every opportunity to illustrate the laws of mechanics and electromagnetism, including what it would take to colonise space! This became his mission, on which he spent the rest of his life after leaving particle physics. I was fortunate to be the course-manager –we had about 10 faculty members teaching it –and we all helped to support him in this theme, along with another important innovation, a sensible programmed learning vehicle we called “the Learning Guides.” Programmed learning materials were a fad then, but not very useful because they just marched the students through a problem step by step. The Learning Guides by contrast first asked the students to do the problem cold turkey. If they couldn’t get the right answer, the guide gave them a set of increasingly detailed “helping questions” they could consult until finally succeeding. More than 50 years later, the Guides are still an important tool in our courses today!
Back to life at Princeton! CP violation was even more the talk of the town when I came back from Germany in 1967. Jim Cronin had done an experiment in ’66 that found a large, unexpected source of CP violation. Specifically, most models predicted that the rates for KL → π+ π- and KL → π0 π0 should be the same, but Jim’s experiment found that the π0 π0 mode was four times larger than π+ π-. This generated hundreds of theoretical papers and all kinds of new proposals for experiments at accelerators, including the one I had come to Princeton to join. Working again with my former thesis advisor Pierre Piroué, we used the Brookhaven AGS to measure and compare the energy spectra of K+ → π+ π+ π- and that of its antiparticle K- → π- π- π+, which was thought to be an attractive place to find a charge asymmetry, which would indicate CP violation.
Stew, you’re talking about E787 at Brookhaven.
No. E-787 came more than 15 years later. This experiment—I can’t even remember its number, began in 1967 shortly after I came back from Hamburg, and produced results in 1971. We had three million events, for those days a very large sample, and were able to set a very stringent limit on CP violation in this channel. But the trouble is, in the middle of this and the other many experiments triggered by Cronin’s result, Cronin thought maybe there was something wrong. To explore the situation, he and his colleague Val Fitch called together a small conference at the PPA to explore the situation. A remarkable photo taken of the attendees includes at least 10 then or future Nobel Prize winners.
Stew, I wonder if you can read the caption for the audio.
Yes, and I can also send the photo to you. It says K Meson Conference, Princeton-Pennsylvania Accelerator. November 3rd and 4th, 1967.
At the conference Jim’s colleague Val Fitch presented the results of an experiment he had just done at the PPA, setting an upper limit on KL → π0 π0 that was much smaller than Jim’s result, and compatible with conventional predictions. This was right at the time we started our experiment, as did many other groups around the world – everyone was looking for the great new source of CP violation, which among other things possibly could explain the matter-antimatter asymmetry of the universe (the original Fitch-Cronin effect was a billion times too small!).
While all this was going on, Cronin redid his experiment at the Brookhaven AGS and found (KL → π0 π0 )/ (KL → π+ π- ) = 1 within statistical uncertainty of a few per cent. His original experiment was wrong!! Maurice Goldhaber, the director of Brookhaven, said, “You know I love Jim and he’s a great physicist, but that was the most expensive mistake in the history of particle physics.” [laugh]
Our experiment flourished, nevertheless. The limit on CP violation we obtained in 300 hours of running lasted for 50 years, before it was superseded by an absolute gigantic effort at CERN, NA-62, which took more than 15 years. The Particle Data Group publishes tables of measurements of every conceivable parameter, and though I don’t know if ours has lasted the longest, but it certainly must be in the top 10. [laugh]
[laugh] Stew, on the social side of things, being a young professor on campus in the late 1960s and early 1970s, what was your experience with student protests and the anti-war movement and all of that?
Oh wow. When I left for Hamburg in March 1966, there were some rumblings about Vietnam but civil rights was still more prominent -- the March on Washington, voting rights bills and so on, and LBJ was still at the peak of popularity and influence. In Hamburg and most of Europe on the other hand, there was uniform outrage at what was happening in Vietnam. I didn’t think too much about it because we were spending most of our time working, but I had great sympathy for the several North Vietnamese I met at DESY, who were terribly worried about their families back home.
When Norma and I came back to Princeton in Sept 1967 we were taken aback by all the changes in the 19 months we’d been away. The whole scene in the U.S. had become unrecognisable and protests were in full force, highlighted by a national moratorium at US Universities to protest the war in Vietnam. There was a university-wide faculty meeting that day, at which various professors gave speeches, including an eloquent one by astrophysicist Martin Schwartzchild. Fortunately, Princeton had a great president then, classicist Robert Goheen, who had been a missionary in India as had been his parents. He was strongly against the war, and in fact ended up on Nixon’s enemies list. He won the day by saying, “Look, we shouldn’t be fighting internally. We should be going out and trying to influence the country. Students should have a chance to participate, and we have to adjust the curriculum to make this possible.”
This idea had traction, and the university calendar was changed to put a break at the end of October, so that students could work on the 1968 election via campaigning, helping with “get out to vote drives,” and things like that. Student protests were almost non-existent because everyone knew the president was on their side. Whereas Harvard, Columbia and Penn had serious protests and even riots, we didn’t have any. There were peaceful protests, of course, but things were so well handled, mainly by Bob Goheen, that we escaped most of the turmoil.
The Vietnam malaise had got so bad by 1969 that I was thinking I couldn’t stand it anymore and might have to go back to Europe. By then we had a second son Ian so a move would be a very big deal, and this thought lay latent until Sam Ting offered me an assistant professor position at MIT, which meant we’d be living in Hamburg again away from “Vietnam.” It was so tempting I accepted it before realizing all the unintended consequences: uprooting the family, putting my wife in a very tough spot – virtually on her own with two babies to take care of; less independence, etc. Luckily, I had not resigned from Princeton when “at the last minute” I decided to stay after all. I called Sam to decline the MIT position, expecting him to be furious with me, as he would have been entirely justified in doing. I’m forever grateful to him for his understanding at that time, and for his friendship and support ever since. Let me tell you – it’s almost as bad to have two jobs as zero jobs! Boy, was I relieved when this was all settled, and I’m certain in retrospect I made the right decision.
Back to the science. I’m curious how close of a witness you were to the discovery of asymptotic freedom and if that was relevant at all to your research.
David Gross, John Schwarz and I became assistant professors in 1969, and a couple of years later I was a member of the graduate admissions committee when we admitted Frank Wilczek. David and John became personal friends, and tried to explain what they were working on. Theoretical models of strong interactions were a dime a dozen in those days, and I for a while I was too stupid to realize the singular importance of David’s and Frank’s work on QCD, nor the prescience of Schwarz’ pioneering in string theory (with regard to Schwarz and string theory I was not alone – John was basically ignored for several years). The deep inelastic scattering experiment at SLAC and subsequent muon experiments at Fermilab and CERN revealed what became known as scaling violations, indicating the presence of gluons as well quarks, in agreement with the predictions of QCD. This led to ever-more-stringent tests, all of which confirmed QCD. Seminars and colloquia at the time were spectacular as Princeton became the center of the theoretical universe. Great students came to Princeton to work with David and Curt Callan, including Ed Witten, Cumrun Vafa, Igor Klebanov, Juan Maldacena and others. At this time the climate shifted to string theory as Witten joined the faculty, and later moved across town to the IAS, to be joined by Seibert and Maldacena. Princeton’s position in string theory was unique, and till Harvard woke up a few years later, they had the playing field to themselves. But the original euphoria of string theory has dimmed somewhat as it became realised there were many string models that could work, and that the energy scales were so vast they couldn’t be probed by experiment. Nevertheless string theory has made profound contributions to mathematics, and remains the only plausible, possible picture for combining particle physics and general relativity. At the time I remember someone, probably Gross, saying “If you don’t like my theory, what’s your theory?”
Stew, when did you start to take on graduate students of your own?
About 1970, ’71.
And what kinds of projects did you have them working on from the beginning?
Everything. The first experiment was a small one and it didn’t work very well. I and a fellow assistant professor Cy Hoffman, who were used to having groups of four or five people, naively proposed a lot more complex experiment at Brookhaven, looking for the rare beta decay of the sigma hyperon, which involved a neutrino and hence a less-than-optimal signature. We did our best but failed to reach the sensitivity needed to make any progress. However, the experiment was invaluable in teaching me and our two outstanding postdocs Mike Witherell and Gary Sanders “the facts of experimental life.” We gave up on that experiment and moved on to Fermilab for an important set of experiments in collaboration with a U. Chicago group led by James Pilcher, studying the production of muon pairs in hadron collisions.
In a long program of three experiments using the Chicago Cyclotron Spectrometer, extending over about 10 years, to measure muon pairs produced in hadronic collisions at ~200GeV, a new energy regime enabled by Fermilab. Less than 6 months after its discovery we measured production of the J/ψ meson and used the Drell-Yan process to measure the quark structure of nucleons, pions and kaons, including QCD effects. In Drell-Yan a quark (antiquark) in the beam hadron would annihilate with an antiquark (quark) in the target hadron, into a virtual photon that decayed immediately to a μ+ μ- pair. By measuring the cross sections as functions of angle, energy and mass of the pairs we could then map out the quark structure of the nucleons and mesons, including the subtle “higher-twist” effect from QCD effect in which gluons affected the dominant Drell-Yan process to change the angular distribution of the pairs. Much of this work was only possible because Fermi’s original cyclotron magnet, which had been moved to the Muon Lab at Fermilab, provided the largest acceptance of any pair spectrometer in the world. An added satisfaction for me was that I could apply knowledge gained with the e+e- pair spectrometer at DESY to this new situation.
You asked about students. Branson’s thesis reported our first results, after which he has had a marvelous career at MIT and U.C. San Diego. Later students Sandro Palestini and Cristina Biino are now at CERN and U. Torino, with major roles in ATLAS and the NA 62 experiment. Chicago students Kathy Newman, Jim Alexander and Chris Adolphsen have all done exceedingly well at Princeton, Cornell, and Fermilab. I am also very proud of the many postdocs and junior faculty at Princeton and Chicago, who went on to outstanding careers. From our original group of 9: Gary Sanders, leader of LIGO and the Thirty Meter Telescope; Jon Thaler, professor at U. Illinois; Kirk McDonald, professor at Princeton; Eli Rosenberg, professor at Iowa State, and Kelby Anderson, senior scientist at U. Chicago. Later notables include Bill Louis, now at Los Alamos, and Wyatt Merritt, U. Chicago.
Stew, what are the origins of the Chicago-Princeton collaboration at Fermilab? How did that get started?
Thanks for asking!
Fermilab turned on in 1972 and the SPEAR e+e- collider at SLAC soon after, to find puzzling anomalies in their early data. As I told you earlier, Jim Cronin moved to Chicago in 1972, mainly because he was building an experiment for Fermilab with my former advisor Pierre Piroué and hated the thought of having to commute, as he had done for so many years to Brookhaven. He said, “I just can’t stand it.” Chicago wasn’t dumb and made Jim an offer he couldn’t refuse; as a result their experiment became known as the Chicago-Princeton collaboration, abbreviated somewhat nostalgically as the “CP collaboration.” They discovered an excess of single prompt muons, i.e. those produced directly in “hard” collisions and not from decays of pions and kaons, that was several times larger than could be accounted for by conventional processes, including decays of vector mesons into lepton pairs, like I had studied at DESY. The Mark I detector at SLAC had also found an anomaly: much higher rates for hadron production than could be explained. Theorists at the time were certain that a 4th quark, charm, would be found, and that it was potentially the source of both single leptons and lepton pairs.
Cronin called me up one day and said, “You and Jim Pilcher, an assistant professor at U. Chicago, should work together.” I replied, “sounds good!” We proposed an experiment to determine whether the anomalous muons were produced in pairs or were single muons from decays of new particles, and a second Chicago-Princeton collaboration was born. This became a wonderful partnership, and we’ve been close friends ever since. Jim P. is a simply-first-rate scientist and colleague, and the fact he’s Canadian and former Princeton student doubtless helped! When we formed our collaboration, to relieve the redundancy with Cronin-Piroué we became the “CP-II” collaboration, or as Leon Lederman jokingly called us, “CP-Junior,” because Pilcher had been Cronin’s student and I had been Piroué’s.
The discovery of the J/ψ by Sam Ting at Brookhaven and Burt Richter at SLAC in November 1974 revealed that charm decays were likely the missing source of prompt leptons, and our experiment proved it! It is amazing that charm wasn’t discovered earlier at Fermilab where, because of its much higher energy than Brookhaven’s, the production rates were at least 100 times higher! In fact two experiments could have, should have discovered charm a year earlier, but failed to do so for different reasons. First, Leon Lederman was building a pair spectrometer precisely to look for particles like the J/ψ ! but deferred the second arm to pursue the anomalous single muons discovered by Cronin and Piroué. Second, there was Fermilab E-98, the experiment ahead of ours in the Chicago-Cyclotron facility, designed to study deep inelastic muon scattering. One leader of the group, Herbert Anderson, was adamant that only muon beams should enter “his” spectrometer – he had been Fermi’s assistant when it was built. His collaborator, Richard Wilson of Harvard, had made earlier muon-pair measurements at Brookhaven and wanted to take a brief run with a pion beam, but was rebuffed. It was a rather grumpy collaboration. I remember Herb’s saying “I will never allow a hadron beam into my beautiful muon spectrometer.” If they had done so, they would’ve found the J/ψ in a couple of days, as we did when we took over the spectrometer a few months after the discovery. What might have been! But Fermilab was redeemed in a sense three years later when Leon then discovered the Upsilon meson. I saw him a couple of hours before he announced the discovery and remember well the Cheshire-cat grin on his face. Everybody wins.
How much time would you spend out in Illinois?
Oh man! Because I was teaching, I had to fly out every week, sometimes even twice a week (no internet or ZOOM!). I was usually able to arrange my classes for Monday- Tuesday-Wednesday, fly out Wednesday night and come back Saturday night for a free Sunday with my family – actually not so free because I was coaching my sons in a youth hockey program. As you can imagine, this was very stressful – even worse than Brookhaven – and I don’t know how my wife put up with it. But it was so exciting because we were going where nobody had gone before – it really lit my fire!
What were some of the principal conclusions of the Fermilab experiments?
Well, for starters we made a complete survey of J/ψ production and explained the production mechanism. Next, we were in the perfect place extensively to survey the Drell-Yan mechanism. One of the first things was, what is the rate? With QCD and color it should be three times the nominal because there are three different-colored quark amplitudes, which we confirmed. We also definitively demonstrated a unique feature of the Drell-Yan process. In the right kinematic region muon-pair production is dominated by quark- antiquark annihilations and proportional to the square of the annihilating quark’s electric charge: (2/3)2 for a π- beam (u u annihilations) vs (1/3) 2 for π+. Then for a target nucleus containing equal numbers of neutrons and protons, and hence u and d quarks, the π- beam should produce 4 times as many pairs as π+, whereas for an ordinary strong interaction symmetry consideration says the rates would be the same. More dramatically even, the μ-pair rate in proton-nucleus collisions should be much smaller than in pion-nucleon collisions, because to first order the proton has no antiquarks to annihilate. We were the first to measure these ratios, finding that the π- beam was producing 100 times more μ pairs than the proton beam. So, this was absolute proof that electromagnetic quark, antiquark annihilation was at work. Other experiments at Fermilab and CERN made useful contributions, but the huge-aperture of the Chicago-Cyclotron magnet allowed us to explore more kinematic regions than anyone else.
Because the energies were higher.
Actually, the energy at CERN was not so different, but our angular acceptance was much superior. In other words, a small-acceptance spectrometer will only be sensitive to somewhat symmetric pairs, where both the μ+ and μ- must have close to the same energy, restricting the decay angle in the virtual photon center of mass to angles close to 90 degrees. But we could capture asymmetric pairs as well, where one muon goes forward with high energy, the other out to the side at low energy, and thus measure a much broader angular range of the decay process. The other pair spectrometers couldn’t do that because their two separate arms allowed no path for the strongly-bending low-energy μ’s.
Stew, how closely related was this to your involvement in the E787 experiment at Brookhaven?
Not at all. E 787 addresses the Standard Model of the electroweak interaction, a different issue. In 1982 Erich Vogt, then director of TRIUMF laboratory in Vancouver, asked me to come for the summer to help with the design of experiments for their proposed ill-fated “KAON factory,” a machine similar to the Brookhaven AGS but with an intensity 100 times higher. I was struck to realize that the field of K decays, so prominent in the 1960’s, had been subject to a long period of, to use Nixonian terms, “benign neglect,” and that it was time to revisit this area of science. Many advances in technology and in theory took place during this interregnum, and in thinking about KAON, I also realized that improvements at the AGS offered a factor of 100 increase in sensitivity over previous experiments!
Before leaving I got together with two of my young Princeton colleagues, Bill Louis and John Greenhalgh, to plan for kaon physics while I was away, with regular communications to see what we could come up with. Two great opportunities appeared immediately. The more attractive of the two, experimentally, was to search for the lepton-number-violating process KL → μ e. Bill and John worked with me at Fermilab, and hence we were all pair-spectrometer experts, and before long were able to design a “perfect” detector. Concurrently we joined forces with a group at Brookhaven, led at the time by Ted Kycia, and a younger physicist Laurie Littenberg to exchange ideas on possible experiments and collaborations. While we were exploring KL → μ e they focused on the decay K+ → π+v v, highly suppressed in the Standard Model, but not completely forbidden – at the time the predicted branching ration was approximately 10-9. By the end of the summer Bill and I decided on K+ → π+v v as the better experiment because it had a well-defined physics goal, whereas as sweet as it was experimentally, KL → μ e would be a terrible long shot, based on overwhelming theoretical evidence that the branching ratio would be unreachably small.
In the early 1970’s Glashow, Illiopolous and Maiani predicted charm by showing that including charm in weak decays would account for the observed suppression of “flavor-changing neutral currents” in several kaon decays. Moreover, K+ → π+v v was unique -- as a purely weak process, the calculations did not depend on knowledge of the structure of the K+, unlike its electromagnetic analogues K+ → π+e e, K+ → π+μ μ , etc. The bad news was that the predicted branching ratio was far beyond the sensitivity of any previous experiment, but we all felt we could achieve it.
While I was out at TRIUMF I persuaded their management to gain experience at Brookhaven to inform their future program and warned them that the AGS still had the potential to improve the sensitivity of several experiments as much as 10,000 times before KAON could even get on the air! As a result Doug Bryman, one of TRIUMF’s most outstanding scientists, joined us along with his powerful group and access to TRIUMF technology.
The fact that we could achieve 10,000 times increase in sensitivity was so exciting! This particular process had barely been looked at, and only by primitive experiments that produced upper limits by vetoing background events instead of performing a blind analysis, thereby reducing their sensitivity by an unknown amount. When I read the thesis it reminded me of the amateurism in the Pipkin QED experiment! They would have thrown the baby out with the bath water, in other words.
John Greenhalgh kept working KL → μ e and carried our design to a group at Stanford who adopted it for E-791, a successful experiment that greatly improved the limit on this process. Our work was therefore not lost, but I think John deserved more recognition than he received.
By spring 1983 our new collaboration of Brookhaven, Princeton and TRIUMF was a going concern, active in detector design and R&D on several new technologies needed for us to succeed, notably a highly-segmented scintillating stopping target and range stack, and massive numbers of transient digitizers. We were unusual, even a throwback to earlier times in using a stopping beam. It proved the key because it enabled powerful techniques to capture “all” the information on an event, like measuring the range, energy, momentum, and observing the decay chains of pions and muons over several microseconds. Nobody thinks of using range measurement in particle physics, but we could do it here because the particles had low enough energies to stop in a reasonable amount of scintillator. In the end we designed the experiment to get 10-11 rejection of background. Unheard of, but it worked!
With Princeton, Brookhaven and TRIUMF we now had three strong groups, and were delighted when Maged Atiya joined us. An assistant professor at Columbia, Maged was an electronics expert and had access to the famous Columbia electronics group. Also, Dan Marlow, an assistant professor at Carnegie Mellon, became involved and wanted to join the collaboration. I then received an annoying phone call from Dan’s boss, who told me Dan could only join if he could join too. We did not want this guy, so I simply hired Dan at Princeton. Dan’s boss is a very nice person, but he should have never said anything like that. Anyhow that’s the way Dan came to Princeton.
The key to making a sensitive background-free measurement of K+ → π+v v to minimize “dead material” in the detector,: i.e. material where a charged particle can lose energy without it being measurable. This was crucial for us, because of the continuum of missing energy carried off by the neutrinos the pion spectrum was also a continuum – no beautiful energy peak, alas. All the backgrounds came from other K decays: 63% K+ → μ+ v, and several decays to pions: ~20% from K+ → π+ π0; another ~7% from K+→ π+ π0 γ, π+ γ γ. So, the whole idea was to reject all other K decays while retaining high efficiency for the signal. To identify π+ and reject μ’s we measured the energy, momentum and range. As a first check, events were rejected unless all three parameters were consistent with a pion. Next, all pion decays also contained one or more photons, which were rejected by crystal-scintillator photon vetoes. Finally, about a quarter of the pions in K+ → π+ v v had higher energy than background pions. So, we have a multi-dimensional space where the π+ signal events would have higher energy, longer range, and higher momentum than π’s from any other K decay.
Muons from K → μ v and K → μ v γ posed another horrible background, against which our weapons of energy, momentum, and range were truly powerful, but we still needed another factor of 100 or so to reach our design sensitivity. To separate pions from muons it was necessary to follow the entire π+ → μ+ → e+ decay chain, which occurs over several microseconds – the π+ lifetime is 25 nanoseconds; the μ+, 2 microseconds. To eliminate pileup, we required nanosecond-scale time resolution over several microseconds, and fine segmentation – way beyond state of the art. Fortunately Maged and Bill Sippach, the genius of the Columbia electronic shop, found a way to mass produce transient digitizers. (Maged shortly after this joined the Brookhaven group.) Each digitizer was equivalent to a nuclear-physics pulse-eight analyzer! We ended up with 2,000 of them and were able to cover every scintillator in the range stack for several microseconds.
In a real event we would see the light from the pion as it came in and stopped, thereby measuring its energy and stopping time. That would be the first pulse. Then with a lifetime of 25 nanoseconds we’d see a second, very small pulse when the π decayed into a muon and finally a third pulse when the muon decayed into the electron. Hence, we could easily separate a stopping pion, which produced three pulses, from a muon, which produced only two. With the granularity of the hundreds of transient digitizers thanks to the Columbia electronics group, we had a weapon to get rid of muons that nobody had had before. Quantitatively gave a suppression of a factor of 1,000 in addition to the kinematic rejection described above.
The above techniques completely took care of muon backgrounds, but decays involving pions still posed a serious problem. Fortunately, all of these processes, for example K+→π+ π0 followed by the immediate decay π0→γ γ, contained photons. In response we built a “hermetic shield” of lead-scintillator photon veto counters, (later replaced by cesium iodide crystals), which provided the needed pion rejection. This succeeded beyond our wildest dreams as we quickly reached 10-8 sensitivity, maybe even pushing 10-9. However, there still wasn’t the pot of gold at the end of the rainbow, so we had to push further via major upgrades of the K beam and the detector. This launched a new R&D program to make better photon detectors and reduce dead material where photons could hide, a better fiber target, and much finer segmentation. A new ultra-light central drift chamber for improved momentum resolution, straw chambers in the absorber for range, and better light collection for energy. (The amount of light in a scintillator determined the energy and the penetration depth determined the range.) Bill Louis was in charge of building the range spectrometer; Peter Meyers headed the design of the photon rejection system, and Dan Marlow designed the electronics. Brookhaven built the magnet, fiber target, computing, and the unprecedented set of transient digitizers to record the decay sequences. TRIUMF built the central tracking chamber. Talk about a well-distributed collaboration!
The electronic transient digitizers, completely live range stack, central tracking drift chamber, and a live-fiber target were all new technological features of E787 With so much exciting new stuff to build it was very easy to recruit students and younger people. Of the students that came out of Princeton Dan Akerib is now a professor at Stanford and major force in the LUX and LZ dark matter experiments. Robert McPherson is a leader of the Canadian ATLAS group at CERN, and Mark Convery is a senior scientist at SLAC. Mats Selen is a well-known professor at U. Illinois who has won national recognition for innovative teaching. A couple of the others have had successful careers in finance. I’m really proud of those students. They joined us only because they liked the physics, but also that they could go down to the lab and do the R&D that was needed. For example, Mark Convery and Rob McPherson together built and tested the prototype for the kind of drift chamber we needed, one that hadn’t been built before. TRIUMF people had designed it, but our students built the prototype and proved that it worked. The fun just went on and on in this seamless collaboration.
For the upgrades, however we realized we needed to strengthen the group, and recruited some very excellent Japanese groups to join us for the second run of E787, and then what became E949.
Stew, what was it about the Japanese, what they were doing at the time that you sought out their collaboration?
Some of them were the people from the Japanese laboratory KEK who had done the primitive experiment we had superseded; they were joined by groups from other universities. They were very good scientists, including the guy whose thesis we deconstructed to inform our design, and had been watching us. They were delighted to join and took on building a splendid photon rejection system for our upgrade, which enabled us to reach the sensitivity we needed to observe the decay, the pot of gold. We found seven events at the standard model level (which the theorists now calculated to be 10 times lower than when we began the experiment!), confirming it and ruling out any new physics.
We all were really pleased with our experiment. It really worked out well and our result held for more than 20 years, until it was recently, but only slightly, superseded by NA62 at CERN. As icing on the cake, the American Physical Society awarded the 2011 W.H.K Panofsky Prize for Experimental Physics to Laurie Littenberg of Brookhaven, Doug Bryman of TRIUMF and me for our leadership of E 787.
Experience with E-787 also set me up for my next activity, the BaBar experiment at PEP-II, the SLAC asymmetric e+e- collider. The physics and much of the technology was quite similar – in effect E-787 was a zero-energy collider, and the GIM mechanism that predicted charm and suppressed K+ → π+ v v was extended by Kobayashi and Maskawa to include the top and bottom quarks (it’s important to note that the c, b, and t quarks had been discovered yet), in what became known as the CKM (Cabibbo -Kobayashi -Maskawa) model. Decays of B mesons via neutral currents were suppressed, and with 6 quarks new interference effects could produce CP violation. Furthermore, calculations based on data from the LEP collider at CERN suggested that large CP violation should be observable. Remarkably the actual idea for the BaBar experiment was proposed by a Princeton student who overlapped with me, a theorist Tony Sanda and, of all people the late Ashton Carter, who left particle physics soon after his PhD and ended up Secretary of Defense in the Obama administration.
The paper of Carter, Bigi and Sanda described how to find CP-violating interference between B0B0 mixing and decays, which became the central focus of the experiments at BaBar and at BELLE, a competing experiment in Japan. These experiments were designed to capture all possible decay modes of the B mesons, to enable searches for what was called direct CP violation, a tiny effect in K decays as we found to our dismay in our old experiment at Brookhaven searching for violation in K± → π±π±π∓, but potentially huge in B decays.
I entered the BaBar experiment in a strange way in 1995. I had been chair of the physics department since 1990 and was continuing to work on E 787. However, by this time E-787 was “on autopilot” increasing the data sample and didn’t need me anymore because Peter Meyers had assumed greater responsibilities and was ready to take over. Dan Marlow had also left, first to work on the supercollider, and later the BELLE experiment. For E787 all the Princeton high-energy group needed was a senior physicist to lead the physics analysis along with students, and with Peter Meyers we were blessed!
On the other hand, it was clear we needed a major new activity to attract outstanding new postdocs and students and to exploit the fabulous technical infrastructure we had at Princeton, so I began to look for new directions We were one of the few universities who had decided to retain the capability to design and construct major experiments in addition to performing the R&D to inform design. We could still build almost anything and do R&D at almost any level, so I could think big.
And Stew, were you involved with BaBar from the beginning?
No, not the beginning. Lead- ups to what became BaBar began in the late 1980’s when I was fully engaged with E 787. I went to some of their meetings but didn’t join. However, thanks to my colleague Kirk McDonald, Princeton was a founding member of the collaboration, and Kirk provided leadership for many years as a member of the BaBar Executive Board.
In 1991 Research Director David Leith and SLAC Director Burt Richter put me on a SLAC committee to review the BaBar R&D program. Then when BaBar formally started in 1993 they moved me to the new BaBar Technical Oversight committee, similar to the Large Hadron Committee at CERN, to evaluate progress and point out problems. We met two or three times a year, and it soon became clear to me that the central tracking drift chamber (DCH) was in trouble. Canada had the sole responsibility for the DCH, and from my E 787 experience with drift chambers, and knowledge of the Canadian scene from E 787 and my summer at TRIUMF, I could see that the project was not converging, and in fact likely to fail. Our Princeton engineer concurred, concluding that the design was unbuildable, and the group was understaffed to carry out a project of this magnitude even with a good design. This supported what I had jocularly dubbed the “Smith theorem:” Canadian time estimates must be multiplied by at least π.
This illustrates a fundamental problem in the Canadian NSERC funding system: to win a grant you have to promise much more than you can deliver.
When I said all this in a meeting Burt Richter almost threw me out of the room.
The DCH project continued to struggle, and in fall 1995 – catastrophe! The Canadian government cut them off completely, leaving BaBar up the creek without a drift chamber with only 2 years to go! They were completely out of business. David McFarlane, the leader of the Canadian group, who was on sabbatical at SLAC at the time, posted the funding-denial notice on his office door along with a sign “Will work for food.” Fortunately, Tom Kirk of Brookhaven and I were on an NSERC review committee and wrote strong letters advising NSERC to restore the funding, which eventually they did but even so their project would never be finished when needed.
One day in January 1996 I got a call from SLAC Research Director David Leith and BaBar Spokesman Dave Hitlin, who asked me to come out to SLAC for a talk. They explained the drift-chamber crisis and asked me if I would take over the project. (Part of their thinking may have been that my involvement would also help the Canadians with their funding.) I decided to jump on board, and soon confirmed my earlier conclusion that the Canadian design would never work. As our Princeton expert engineer put it, “the design of the carbon fiber end plates is impossible. You simply can’t build that.” It normally takes more than three years to design and build this kind of drift chamber, and there was no money, no feasible way forward without going back to the drawing board for a new design. Fortunately, at this time BaBar made technological choices that freed up several wonderful groups from France and Italy, and the US DOE agreed to contribute major additional funding. I said OK, we can build one that will work, with the Italians, French, the resuscitated Canadians, and greatly expanded DOE funding for the US groups at Princeton, U. Colorado, Colorado State, U. Maryland, SLAC and Lawrence Berkeley Lab!
To a large extent the Canadian group, mainly centered at McGill University and the University of British Columbia, was in shambles. They had hoped to attract other groups at Universities of Victoria, Alberta, and others but these groups were not interested because they did not have confidence in the leadership and felt the project had been oversold (which of course it was, and had to be, to get the money). I had to decide whether simply to take over as leader, or to partner with the Canadian leader. I said “Let me think about it. I’ll go talk to people,” and went up to TRIUMF laboratory in Vancouver BC, which fortunately had a fabulous director –Alan Astbury -- an eminent physicist and superb scientific leader and manager. Alan knew what was going on and said TRIUMF would be the host laboratory for the DCH construction, but only if I agreed to lead the project. This strong support from TRIUMF made the Canadian contribution viable, and thanks to Alan we successfully constructed the chamber at TRIUMF in “record time,” made possible by the state-of-the-art robotic system the Italians designed a built to string the chamber. This was a daunting task because there were 28,000 extremely fragile wires, all of which had to be at a uniform tension. (Roughly there were 7000 20-micron tungsten signal wires and 21000 80-micron aluminum field wires.) The French built superb gas and safety systems and provided leadership in computing, the Colorado groups handled the sensitive high-voltage feed-throughs, which had been the Achilles heel bane of many previous drift chambers; SLAC and LBNL were responsible for the novel electronics, which digitized the signals directly on the end-plates and transmitted the information over optical fibers. (in all previous chambers the signals were carried from the chamber in individual coaxial cables – there was no space for us to go this way so we had to take the risk. Dave Coupal and Dave Nelson at SLAC, and Michael Levi at LBNL deserve knighthoods for pulling this off!)
We took responsibility at Princeton for two of the most urgent and technologically challenging items: the aluminum end plates in which 28,000 holes have to be drilled to 10-micron precision; and the ethereal wire: very weird stuff! Thinner than human hair to achieve high fields on the surface, this wire easily breaks and/or sags under tension, problems that must be tightly controlled for stable operation and position resolution. We knew that recent drift chambers, for example the CLEO chamber at Cornell, had suffered numerous broken wires, which usually became tangled, completely disabling the chamber. They were also extremely difficult to remove. The wire manufacturers had lost their magic touch, alas, so we launched an R&D program at Princeton in partnership with the vendors to get the right properties, finding that the key is to apply just the right amount of tempering to make the wire strong enough not to break, and stiff enough not to “creep” under tension. We only succeeded because were blessed to have a highly-expert technical physicist at Princeton then, Changguo Lu, to perform the R&D. Lu had first come to us for a 2-year visit in 1979, supported by T. D. Lee’s program to help Chinese physicists catch up after 15 years of suffering under the cultural revolution. Then in 1989 Lu requested to visit us again, and of course we agreed. He had sent us his flight information when Tianmin Square intervened and we assumed he’d never make it, but Chinese bureaucracy saved the day – he had all his papers before the massacre and encountered no problems in getting on the plane. We later got his family out two years later, and they are still here.
As the project proceeded, we melded into a well-balanced collaboration where everybody had critical and exciting responsibilities. We loved what we were doing and knew that BaBar’s destiny depended on us to complete a working chamber on schedule. No-one had time to grumble about stupid things. To make a long story short, all the components arrived at TRIUMF on schedule in the spring of 1997, the stringing went smoothly during the summer and fall, and we then shipped the chamber to SLAC. The whole experience had been exciting and enjoyable, and our success became a notable event in Canada. In a way we were sad it was over: amusingly, I was quoted in the Toronto Star to say: “I cried when we had to put it in the truck.”
The SLAC group took over to install the electronics and commission the online data acquisition system. This was no mean feat because as mentioned above this was the first large drift chamber to have its data digitized right on the endplate and transmitted via optical fibers. (In all previous chambers the signal from each sense wire was carried out by a coaxial cable – we simply had no room for 7000 cables so the state of the art at the time was not an option.) Six months in the lab at SLAC did the job, with the DCH operating on cosmic rays in time for Burt Richter to show our results in his talk at the International Conference on High Energy Physics (ICHEP) in Vancouver, alerting the HEP community that BaBar was back on schedule.
We were able to build the chamber in two years only by learning from others, especially from the Cornell group, who generously shared their experience and made several crucial suggestions. Even the ill-fated early DCH project was useful in that it produced an elegant small-cell configuration with high resolution, and exposed a lot of mistakes so we would not repeat them.
Stew, from your vantage point as an outsider looking in, to what extent did BaBar represent really a newer direction for SLAC?
Completely! Previously there were the well supported in-house groups, and the visiting “outhouse” groups, who were almost completely dependent on SLAC collaborators and their resources. On the other hand, more than 50% of the BaBar detector was built and paid for by non-SLAC groups. The B factory was a $300 million project: $200 M for the accelerator, $100 M for the detector. At least $50 M for the detector came from other countries, and another $10M or so from US universities and National Laboratories. Most revealing, the beyond-state-of the art silicon tracker was entirely designed and constructed outside SLAC by Berkeley and Pisa groups. Similarly, the drift chamber cost about $10M, more than half of which was funded outside SLAC. I had control of ~$2M at Princeton, other US universities had another million, the French and Italians ~$1M each. And the Canadians provided all the infrastructure for stringing, which would have been at least twice as expensive at SLAC.
So yeah, BaBar was a revolution for SLAC, and they dealt with it extremely well. I’d give huge credit to the late David Leith, SLAC research director throughout the BaBar construction, who realized that other countries and major US groups would not provide resources without a full-fledged partnership in all phases of the experiment. Major responsibilities needed to be allocated on merit, stakeholders should share in deciding how resources were allocated and overseeing that they were well-used. To accomplish this he set up the BaBar International Finance Committee (IFC), composed of funding-agency representatives from the several countries making significant contributions to BaBar: the US, Canada, Italy, the UK – and two from France because they had two very competitive funding agencies. These people worked extremely well together, and when problems emerged, they invariably took the high-road to find a solution.
On a personal note, I finished 8 years as Chair of Princeton’s physics department in June 1998 and took a year sabbatical at SLAC to lead the commissioning of the drift chamber and prepare for physics. BaBar was blessed with an abundance of outstanding physicists to build the detector, but Jon Dorfan was the singular driving force that brought the experiment home on time. If there were a single person without whom we wouldn’t have made it, it would be Jon. I was therefore shocked when he came into my office a few weeks into my sabbatical and told me “Stew, I just want you to know that Burt (Richter) is stepping down and I’ll be taking over as the new SLAC director.” Oh shit.
And then a month later he came again and said “You know, the experiment is dead in the water. It’s just not working.” I said, “Oh my god. Well, I’m going to be here for another few months and maybe could help,” And he replied, “Are you volunteering?”
I said, “Yeah, I guess so,” and without any to-do took over from him as Technical Coordinator (TC). This was in the fall of 1998, when the subdetectors were arriving from where they had been constructed to be inserted into the BaBar detector. The wonderful quartz bars for the DIRC, the unique BaBar Cherenkov detector, were coming from Boeing, the Italians sent over the silicon detector, the CsI crystals for the electromagnetic calorimeter arrived from China and Russia, and of course we had our drift chamber to install. The place was crawling with engineers, technicians, postdocs and students.
In my two years as TC, I oversaw all the integration, commissioning and early data taking and was proud of all we had accomplished. Halfway through in the fall of 1999, I was already planning my return to Princeton when Dorfan offered me the job of SLAC deputy director and professor at Stanford.
Did you seriously consider that?
I thought I was going to do it. I really thought I was going to go, as I never thought Princeton would match his generous offer. To my surprise, Princeton president Harold Shapiro called me in and asked me what it would it take to keep me at Princeton. I didn’t want much of anything new, just a matching of salary and a guarantee of strong university support for High Energy Physics. He said “sure,” and after a couple of stressful weeks considering what to do, once again I decided to stay at Princeton. A significant factor was the Silicon Valley housing situation. We had a very nice place in Princeton and had lived in it for 20 years, so my wife and I felt we’d need to find a house like ours in a neighborhood like ours.”
Right in the peak of the dot com boom.
There was a factor of 3 in housing costs, which was too much to deal with even though Stanford was incredibly generous in helping, along with the fact I wanted to keep working full time on the experiment. And so, we stayed.
Dorfan took me to lunch to discuss what to do and told me he was worried about the Spokesperson situation, now that the experiment was operating and needed on-site leadership. The outstanding founding spokesperson Dave Hitlin had spectacularly led the collaboration in securing funding and organizing the design and construction. Dave’s base was Caltech, from where he traveled as needed to interact with the various groups in Europe, Canada and the US. Everything worked fine until all the action descended on SLAC as we installed and commissioned the detector. Dave’s routine was to come up to SLAC three days a week, which was fine until the issues involved in the commissioning became overpowering and needed 24/7 attention, including weekends. Most of this devolved on me because Dave wasn’t able to be resident on site. I really felt sorry for him because he was between a rock and a hard place, because for various reasons he wasn’t able to be at SLAC full time. Dorfan ended our lunch by saying “Okay. If you’re not going to take the Deputy Director job, then you’ve got to run to be spokesman.” I replied that I would never compete against Dave, but if the collaboration asked me I’d be willing to be considered. And so, they did. To make a long story short they chose me to be spokesman. This was extremely hard on Dave, who wanted to continue, especially since it was the time when the first data was about to come out. It was very hard on both of us, but we survived.
My job was to put together an efficient functioning Senior Management Team, and to succeed I knew it had to be small. (The previous Spokesperson had a management meeting of 15 people that was so ineffective I didn’t go to its meetings when I was Technical Coordinator.) For my team I recruited and/or the collaboration elected outstanding scientific leaders to represent our “three branches of government.” Computing (Jim Smith, U. Colorado), Physics Analysis (Gautier Hamel de Monchenault, CEA-SACLAY, who went on to become deputy spokesperson for the CMS experiment at CERN), and Technical Coordination (Yannis Karyotakis, Annecy, who became Director of the Laboratory of Particle Physics at Annecy, France). This was an outstanding team, but with a glaring hole: the lack of a SLAC person. To fill it we created a Senior Advisor position and were extremely fortunate that Bill Wisniewski accepted it (Bill went on to serve for many years as an outstanding Technical Coordinator.)
We met weekly to address the critical issues and problems standing in the way of routine data taking. John Dorfan had really taught me management 101: as he told us when he was Technical Coordinator: “You tell me your problem and then I have to find you what you need, or fire you and find someone who is able to solve the problem.”
At our first meeting we quickly found there were “elephants in the room” in addition to our benevolent mascot BaBar: online computing and database. The experiment depended on prompt reconstruction of the raw data before storing it, and the online system was completely inadequate to handle the onslaught. Equally serious, the proprietary database chosen at the beginning of the experiment, Objectivity, did not live up to the claims of the vendor and was much too slow for efficient offline analysis. We needed a hard-core computing expert here to help us make the difficult decisions necessary to get out of the crisis we inherited and added the Deputy Computing Coordinator Stephen Gowdy of Berkeley Lab to the management team. Stephen not only had the special expertise, but also enjoyed the respect of all the computing staff at SLAC and around the collaboration.
Management à la Dorfan became our model as we had to make significant changes, which were very painful. Most serious, we had to take the computing system apart and replace it, which could only be done by inserting new people and removing people who had been working hard for years. My experience with the drift chamber proved valuable here, and everything worked out well in the end.
Back to the IFC. This powerful group literally saved the experiment! For example, shortly after I became Spokesperson in 2000, we realized we had grossly underestimated the amount of computing required -- by several million bucks! SLAC was responsible for computing but unable to pay for this unwelcome addition, and initially asked the non-US countries to send money to buy computers for SLAC. All the IFC members refused, but French IFC member Guy Wormser miraculously saved the day, saying “how about this? We have a huge computing center in Grenoble for the CERN LHC data, but the LHC is several years late. If you can figure out how to use it, we shall make it available as part of our contribution to the BaBar common fund.” This common fund, assessed to each country proportional to the number of PhDs working on BaBar, covered basic items to operate the experiment. Examples include cryogenic fluids, gases for detectors, computing professionals, etc. Countries could contribute their share as cash or in kind. At that time a common fund was a new concept in particle physics, as was the IFC, so it took a while to converge on a fair and unanimously-accepted process. We first had to agree on which costs were allowable. The experiment would then make funding requests and the IFC would decide whether to accept them.
The French proposed to get common-fund credit for their contribution of computing, and this was immediately approved. Computing was added into the common fund budget and an algorithm worked out to determine the value of a computing center. Almost immediately the Italians, British and Canadians also made some of their facilities available, so we were now swimming in computing. But distributed computing generated a new serious problem: how to transfer the data around the world. Experts in France and at SLAC came up with a revolutionary solution – the BaBar Grid – the first of its kind in particle physics and beyond. The Grid and the immense computing resources it enabled quickly solved our computing shortage, without adding complexity to users. People could submit a job using a particular data set without having to know where the data was stored or processed. The answer would just come back as if it had been done locally. Needless to say, it took a lot of work and skill to bring all this into smooth operation. We struggled a bit as it was being developed, and it arrived just in time to deal with the increasing luminosity. It was simply brilliant --without the Grid we were absolutely stuck – there was nothing we could do. There was no way we could have found enough money to buy all the computing we needed, but even if we had (and this wasn’t widely realized), there would have been gridlock because the data-transmission capability into SLAC would not have been able to handle all the traffic from BaBar users around the world (and we’d have been in a similar pickle if everybody came to SLAC to compute). The key was to distribute the data storage and computing power, as has since been done on a much grander scale by CERN for the LHC.
Stew, I’m curious if as spokesperson you dealt much directly with the DOE.
Close communication with the DOE was essential, and it was a pleasure because they invariably gave us strong support. During the construction and commissioning phases their semi-annual Technical Reviews, fondly known as “Lehman Reviews,” provided the tough love we needed to make progress, and the DOE member of our IFC, Associate Director for HEP John O’Fallon, could not have been more helpful. Furthermore, for those of us in universities the support from P. K. Williams was superb.
By spring 2001 BaBar was digesting the increasing flood of data from PEP-II and preparing to publish our first decisive results. DOE told me they were delighted by our success and asked us to communicate this broadly. In a sense we provided cover for Fermilab, which was behind schedule and having problems increasing the luminosity of the Tevatron, and consequently had little to show at conferences, and perhaps more importantly at the thrice-yearly meetings of DOE’s High-Energy Physics Advisory Panel HEPAP, where there were always several reporters and DOE needed to show progress and excitement. As the only game in town, BaBar was featured at every HEPAP meeting during this period, and it was my job to present important new results for the panel and the reporters.
The BaBar detector itself performed wonderfully, except for the muon system, the serious weaknesses of which were revealed during the commissioning run in 1999. It had to be replaced as soon as practical, so I decided to commission a new project, and to lead it once my term as spokesperson was completed.
Stew, on that point, what were some of the key technical challenges with the muon detector upgrade?
Before discussing the muon detector, let me return to computing, a much more serious problem. To achieve the required resolution the detector had to be continuously calibrated in real time because the conditions were constantly changing, especially in the silicon detector, where we had to maintain resolution on the micron scale as variable beam conditions caused it to expand and contract. This required having an up-to-date conditions database that could calibrate the data in real time as it was being put into storage, as it would be unacceptably resource-intensive to process it off-line from storage in a timely manner. And most important, prompt reconstruction was the only way to tell if all the detector elements were working up to specifications.
The original system simply saturated once the machine luminosity and event rate went up. It just froze. As a first step, instead of processing events one after another in series we divided the data among three computers in parallel to prevent a problematic event from causing a traffic jam. This improvement got us through the first year and allowed us to achieve our goal of observing CP violation, two weeks ahead of our competition in Japan. I loved that!
[laugh] You scooped them.
You scooped them.
Yes we did. We scooped them. And I’ll never forget the night when we unblinded the data. There were four of us, and we knew something no one else knew – truly cool! And it got better. Once BaBar got over its technological teething problems, mainly in computing, our set of outstanding physicists unleashed all the power of the detector on physics, resulting in hundreds of new results. I’m particularly proud of the Princeton group and Jim Olsen, who measured the largest CP violation ever seen, a 10 percent asymmetry in the decay B0 → K± π∓, ~ one hundred thousand times larger than the original Fitch-Cronin discovery in 1964!
Now to return to the muon detector. The commissioning data run in 1999 revealed serious problems. Based on resistive plate chambers (RPCs), a novel technology invented in Italy, the system should have been great but its capability was only marginal because of faulty manufacture and inadequate quality assurance. The RPCs consisted of putting ~10,000 volts between two flat plates~2 m2 in area with a gap of 1 cm. The plates were coated with linseed oil to make the surfaces smooth and to absorb the UV light produced when a charged particle ionizes the gas and generates a signal. It turned out that the muon group had tested the chambers under normal laboratory conditions, but not under the actual “battle conditions” they would encounter at SLAC,” where the experimental hall was not air conditioned. Soon into the run we encountered a classic Palo Alto heatwave: temperatures up to 37 C lasting approximately four days.
The chamber performance continued to deteriorate but nobody could figure out what was going on. It just got worse and worse, so we shut them off to find the problem. Fortunately, Changguo Lu came to the rescue. I asked him to take a look and he quickly found the cause. The linseed oil had not been completely cured and could flow under the electric forces within the chamber, to produce local “short-circuits.” Lu described the structures as stalactites and stalagmites.
Just like the salts in a cave, a point of oil would form and from there on it was positive feedback: the longer the point the more current it would draw, culminating in little tubes that shorted out the chamber locally, resulting in circular dead regions. It looked like a bad case of Lyme disease, or nummular excema! We were able to prevent further damage by greatly increasing the cooling, but in spite of several thrusts of R&D we couldn’t correct the damage already done. As a result, BaBar had to struggle for years with a muon detector that was not very good. Luckily our expert physics analysts found ways to extract enough information on muons to save most of the physics.
As my term as Spokesperson was coming to an end in the summer of 2002, I decided we needed to replace this ailing muon system, and convened workshops to come up with a new design. Three technologies were considered, converging on limited streamer tubes (LSTs). We formed a powerful new US-Italy group to take on this project, containing many of the drift-chamber people and two very strong new groups from Ferrara and Ohio State.
For several years the B Factories at SLAC and KEK-Japan were the major new experiments in High-Energy physics and attracted the best young scientists. Twenty years later a large number of BaBar alumni have prominent roles in ATLAS, CMS and LHCb, the three LHC particle-physics experiments.
And Stew, you really never left BaBar.
You are right. When I returned to Princeton in 2003, I badly needed an assistant professor to bring my group up to critical mass. As spokesperson I was in the perfect position to see all the potential candidates. Two people stood out and I got the department to allow me to make offers to both of them if I wanted, but I quickly settled on one of them, Jim Olsen, who had burst on the scene like a supernova. He had only been a postdoc for one year at the University of Maryland, but it was clear to me he was the one. It was too early for him to take on the responsibilities of a faculty member, but I knew if I waited a year, I’d have to compete with many universities for him. We therefore used a method we had done recently in another case while I was department chair. We offered Jim the job a year from then to let him gain experience and complete his postdoc work before his tenure clock started ticking. Jim knew he was in a strong position and asked me about the possibilities for promotion to tenure. Until recently the odds were very low because we hired a large number of assistant professors under a business model in which most of them would leave for positions at other universities. This gave us a reputation that tenure was almost impossible, but that business model didn’t work anymore. Instead we now had a much smaller number of assistant professors, and decided on their tenure based mainly on merit, along with strategic departmental priorities and needs. I explained to Jim that colloquially, tenure was the product of “matrix element times phase space,” and promised him that if he had the matrix element, if necessary I would retire and provide the phase space. He said, “Okay. It’s a deal” and as time went on his matrix element became larger and larger and it looked like I might have to retire, but fortunately for me circumstances intervened.
By the time I came back to Princeton, between technical coordinator and spokesperson I’d had a total of four years of leave, whereas Princeton didn’t allow more than two-year leaves. The only reasons I was allowed to do this were that the leave came in several parts, and that otherwise I would have left for SLAC: the initial year was a normal sabbatical, the next year so I could become Technical Coordinator, and the last two years to be Spokesperson.
I had been granted this special dispensation by president Harold Shapiro as part of my retention package, but he had been replaced by the next president, Shirley Tilghman by the time I returned to Princeton in December 2002. Well, Harold and Shirley were both Canadians and the reasons for my long leave were compelling, so I was not expecting any fallout when I got a call from Shirley, who asked how I got away with this. I told her the circumstances and suggested she ask Harold to confirm. And so she did, and then called me back to say “Okay. But you owe me.” She asked me to be Dean of the Graduate School, but after a lot of thought I turned this down too for the same reason – BaBar.
Princeton was the only major university that did not have a full-time senior academic research officer. Astoundingly in retrospect, the research administration consisted only of the grants and contracts office reporting directly to the provost, with a faculty committee, the University Research Board (URB) to advise on which grant proposals should be approved for submission to the funding agencies. Decisions on university support, dealing with increasing problems with compliance with all the government regulations, fundraising from and partnerships with corporations and foundations were all being run by bureaucrats, who were very good at what they did but did not understand the ways of science or have any connections to the Washington science structure.
In 2005 President Tilghman asked me to become Chair of the URB. By this time the BaBar muon upgrade was well underway, and Jim Olsen had grown even more than I had hoped, so moving half time into administration made good sense now. And in many ways this was the perfect job: half time, no budget and no staff! The job also seemed very interesting and exciting, so I enthusiastically accepted.
All this happened when Jim, me, three postdocs and three students were still actively working on BaBar. The muon system upgrade was a huge success. It worked beautifully and everybody was thrilled looking forward to our next run. But then, I don’t know if you know, SLAC had a terrible accident in which a contractor technician almost died from being seriously burned and disabled by a 440-volt arc flash.
Raymond Orbach, Director of the Office of Science at DOE immediately shut SLAC down for nearly six months while investigations took place to understand how this could have happened. The old accelerator never completely recovered its luminosity, and to make a long story short BaBar struggled on for a couple of years before being canceled as a victim of the 2008 financial crisis. As a result, we only got a little over a year of data for all our work on the muon system and things weren’t as exciting because everything had been based on the accelerator achieving ever-higher luminosity, enabling us to double our data every two or three years. When it became clear we weren’t going to be able to do that anymore, Jim and I decided to phase out of BaBar when the students graduated and the postdocs moved on. But boy, it was so good while it lasted!
2006 was a transformational year for me and for Jim. I soon found the URB to be a full-time job, and Jim had moved over to CMS. And boy, was he the right guy. I mean he went right up the flagpole in CMS, where he is now deputy spokesperson. They recently suggested he should run for spokesperson, but his family situation won’t allow it at this time. I also counseled him this wasn’t a good time, because there’s no data coming. What you want to do is become spokesman when the last run starts, so that you’ll be there when CMS starts making whatever discoveries are left. That’s what he’s doing, along with serving as associate chair of the physics department. He’s just one of these guys who, if you want something done, give it to a busy person. [Aug 2023 note: Jim just became chair of the physics department!]
[laugh] Stew, to clarify, when you were named chair of the university research board in 2005, were you the inaugural chair? Or what exactly did that mean?
No, the URB was a long-standing group of 7 faculty members including the chair, who had the rank of dean. It was a fine club to join, my predecessors all being distinguished scientists, including Harry Smyth, Lyman Spitzer, Sir Robert May, Sam Treiman and Will Happer.
It was well known that our Research Administration was highly fragmented, with a non-academic financial person in control of tech transfer and compliance, and the Development Office responsible for Corporate and Foundation Relations. As I had been, the faculty were unhappy with this arrangement, so the Provost Chris Eisgruber (now the President) and I commissioned a study of Research Administration by external expert consultants to study our situation and make recommendations. Their senior person and I visited several of our peer institutions to see how they were organized, and it immediately became clear to us that Princeton was seriously behind the times. The consultants presented a proposal for change and the trustees adopted it. The URB Chair position would become Dean for Research, a full-time job, with grants and contracts, corporate and foundation relations, and technology transfer people now reporting to me, instead of to non-faculty staff. Once again I had three branches of government!
We quickly came up to speed and this model worked smoothly as far as it went. But then a “slumbering nightmare:” laboratory animal research, especially with non-human primates, which had essentially been unregulated. And we had a lot of animal research in biology and non-human primate research in the psychology department led in large part by very senior scientists, excellent people to be sure but many of whom operated their labs much too informally to be compliant with current regulations. There was only one part-time veterinarian, charged with keeping the animals healthy and ensuring compliance. I soon learned that her efforts to bring compliance were ignored, causing distrust and putting the university in constant danger of the USDA coming in and busting us with big fines.
I said “Look. Why don’t we take this over this and bring it into the modern world?” Easier said than done, especially recruiting a good veterinarian whom the scientists could respect. The first two years were extremely painful, as we were only able to attract part-time veterinarians. Then fortunately for us, the big pharma companies were getting out of R&D, buying small companies instead of doing it themselves. Whereas previously we couldn’t compete with the likes of Merck, BMS or Biogen. all of a sudden these vets were looking for jobs and we landed an absolutely superb one. To illustrate how critical this appointment was, shortly after she joined us Harvard ended up in big trouble with their non-human primates and asked her to lead the committee to get them out of trouble. We suddenly emerged from being terrible to being first-rate just by having the right person. I had to fight hard to convince the university to meet her salary needs, but once again it proved “you get what you pay for.” This reminded me of when leading scientists were debunking Star Wars and a stupid general countered that “two second-rate scientists are equal to one first-rate scientist.”
Yeah. So, I continued as Dean for eight years, the last two or three of which by comparison were smooth sailing.
Stew, given the fact that it was a new position, what were some of the benchmarks that you established to know if you were on the right track? What was Princeton expecting from you?
First off, they wanted the faculty to stop complaining, and rightly so, that they needed support from the university rather than bureaucratic reasons why things couldn’t be done. For example, a transparent and smooth process to establish tech transfer. We had one of the country’s best tech transfer people, John Ritter, a legendary person who our competitors would have loved to steal from us. John was completely frustrated by having to report to the grants and contracts officers, who didn’t understand the faculty members nor the opportunities and risks, so the first thing I did as Dean was to promote him and have him report directly to me. This soon triggered his former boss to leave (“improving both places” so to speak), allowing me to find a replacement who was into serving the faculty.
Our new team was functioning smoothly and well, but the increasing demands of ensuring compliance and managing conflicts of interest became too much of a load for the veterinarian and our other senior management team. After a prolonged discussion with the University’s Executive VP, I was able to add a new division: Research Integrity and Assurance (RIA) and recruit a senior veterinarian from Merck to lead it. I’m most proud that together we found a way to support a new Catalysis Center in the Chemistry department for David MacMillan (who later won the 2021 Nobel Prize in Chemistry) that has produced important target molecules for pharmacy while serving as a facility to faculty in Molecular Biology as well.
Another problem I soon identified was the need for seed money to enable PI’s to produce some preliminary research to show when applying for grants. As a first step I asked the provost to help and was thrilled when he provided a couple of million bucks a year, and shortly thereafter, eminent alumnus and Google CEO Eric Schmidt and his wife endowed a $25-million fund for aggressive, high-risk research. We were now able to do more than just tell PI’s to dot those i’s and cross those t’s! My last two years were great fun, as we were now able to support great new ideas and tech transfer, and with the new veterinarian, grants and contracts officer and RIA division compliance went from nightmare to dream. I called it one-stop shopping – faculty now felt welcome to come over to the dean’s office to discuss anything research-related with me or my deputy, and things would happen! With the aggregation of the existing fragmented offices and new additions we had come a long way from when I took the URB Chair position in 2005, which as I jokingly told Chris Eisgruber was the perfect job: no staff and no budget!
By the time we brought everything under this umbrella the staff numbered over 60 and the budget was in the millions, long overdue. If I could be a bit immodest, this was a great success. As I often put it, “after all these years every other university is finally in step with Princeton.”
You had a successor, or you were the only person in this role?
No. I had signed up for an initial 5-year term and a 3-year renewal. After eight years we knew there was going to be a new president, and another big job though that was lacking was University oversight of the Plasma Lab. Previously the PPPL director reported to the provost, who had a thousand things to do, with the result that university oversight consisted of quarterly one-hour meetings of the PPPL director with the provost and URB chair, at which he would give a PowerPoint presentation on the science. Little attention was paid to operations problems or relations with the funding agency, the US Department of Energy. In 2008 DOE decided to put the PPPL contract out for bids because they were dissatisfied with Princeton’s performance as contractor, and the lab was experiencing technical problems and a major cost overrun on a new stellarator project. As is DOE’s practice, they demanded that the director’s head should roll even though he was a great scientist and was doing his best with an inadequate budget. The head of the Office of Science called our president to tell her “If you don’t change the director, don’t even submit a proposal.” (What had pissed DOE off the most was the failure of the stellarator project, after ~$100M had been spent and severe delays had accumulated.)
So, one of my biggest tasks as dean was to organize the PPPL proposal preparation and submission, a 2-year multimillion dollar project. We had to hire expensive consultants do all kinds of work to jump through all the intricate bureaucratic hoops required by the Request for Proposal. Fortunately, we had PPPL people who were able to do many of the things that we would have had to pay consultants for, and I had a whole nice office building we could use (it was not allowed to use PPPL space for this) so we didn’t have to rent space. We found an outstanding new director, Stewart Prager from the University of Wisconsin. Prager, myself and Bill Brinkman, who had recently retired from Bell Labs to come to Princeton, and I had asked to help us, then recruited a 12-member Advisory Board to satisfy one of DOE’s requirements. In every case our first choice accepted, giving us the leading experts in the world on all phases of plasma physics, engineering and project management.
DOE dragged out the decision on the contract till literally the last two days of the Obama administration, when I happened to be in Japan working to establish partnerships with Japanese laboratories. I had just returned to the hotel about midnight, very tired after an exquisite Japanese dinner, if you know what those are like, and checked my email to find a message from the DOE telling me to call the Director of the Office of Science Ray Orbach immediately. To my horror my cell phone didn’t work from Japan, but I was able to call him by quickly learning to Skype. He demanded to speak to President Tilghman to announce the decision on the contract – no-one else would do. I said I’d have her call him but found she was on a plane to San Francisco for her Google Board meeting. I explained this to the DOE person who answered but Orbach insisted, threatening “I’m not going to approve the contract before talking with president Tilghman, and if I can’t do it before we’re out of office you’re going to have to start all over again.” And by now it was 1AM in Japan.
Fortunately I knew a Princeton alumnus in the communications group of the office of science and called him to see if he could help. I explained that the trustees had delegated the authority for PPPL issues to the provost and me, and that the provost (now president) Chris Eisgruber speaks for the president when she’s out of town, and is a really good guy: Rhodes scholar, Princeton physics major, blah, blah, blah, blah, blah. My friend was able to get in to see Orbach and call me back with a possible solution: Orbach wouldn’t call the provost, but “if the provost called Dr. Orbach, Orbach would consider speaking to him.” Chris calls, to be put on hold for at least ten minutes, when finally, Orbach reluctantly gave Chris DOE’s approval. Without my serendipitous connection I wonder if Orbach would have had to relent. Thank god we didn’t have to find out! But then Orbach made another demand: “The Secretary of Energy Sam Bodman has to meet your proposed director in person to approve him, and tomorrow’s the last day,” so about 2AM I called Stewart Prager, who fortunately was in Princeton, and told him to get on a train to Washington ASAP. When he got to DOE the place was swarming with moving vans, and movers were actually in the process of removing stuff from Bodman’s office, but the official handshake took place and DOE renewed our contract for 5 years. All’s well that ends well -- an interesting trip to Japan! [laugh]. More about Japan later.
For the next 5 years I was the responsible university officer for the lab, with the director reporting to me. This posed a problem in 2013 when it was time for a new dean, as the effort to manage PPPL had increased a lot and there was concern it might deter some of the candidates for my replacement. The solution was to create a new half-time position, University Vice President for PPPL, which I would take on for 2 years and then, after a year’s leave would retire in July 2016. When finding my successor took longer than planned, I agreed to stay on until my replacement came on board in April 2016 and to retire in May 2017. I wish to acknowledge the university’s outstanding generosity to me by supporting the other half of my salary for 3 years as paid leaves, as well as my final year’s leave at full salary.
And the plasma physics laboratory is doing well?
Things were going quite smoothly till just before I retired, when the lab’s flagship project, a major upgrade of the NSTX (National Spherical Torus Experiment) suffered a serious electrical short, followed by further problems with the new magnets a few months later. Together these failures exposed the lab’s major weaknesses – weak engineering and inadequate funding. The best engineers are attracted to the most exciting projects, and alas PPPL’s projects had not been at the top of the list for many years. US fusion research was not funded very well in general and, more seriously for PPPL, its budget was simply too small to match its responsibilities. The problems were exacerbated by the unhelpful head of the DOE Office of Fusion Energy Sciences, who seemed to have it in for Princeton. Not only were the funds for NSTX inadequate, but also, he drastically cut the lab’s infrastructure and maintenance budgets, even when this color of money was available. The lab’s unfortunate recourse was to take shortcuts in design and engineering, resulting in magnets failing, requiring them basically to start over and in the finest DOE tradition, for Princeton to fire and replace the director.
So, they’re now having to redesign and rebuild NSTX, and I think they’re finally on the right track. And the best thing is, after a year or so of fibrillation the university and my successor serendipitously found a great new director, Sir Stephen Cowley, who had led the UK fusion program for many years, but now wanted to come to the US. Steve knew the lab well as he had been serving on our advisory board all this time and had done his PhD work there. Unlike anybody else he had the gravitas to melt DOEs heart and persuaded them to give the lab what it needs, even though seven or eight years overdue. I think the lab’s future is quite bright because DOE is now letting them broaden into other related fields where their infrastructure will help. Previously it was always a feast or a famine: building a huge project, or once it’s finished, operating it for 20 years without major changes. You can trace all the failures to this cycle, which made it impossible for the lab to retain the best engineers during the long periods between exciting projects.
[laugh] Well, Stew, we already talked about your recent interests at the very beginning of our talk. So, for the end of our talk, now that we’ve gotten all the way up to the present in the narrative, I’d like to ask a few broadly retrospective questions about your career. And then, we’ll wrap up looking to the future. So, one thing—
Yeah. That’s good. I’ve got to be out by about quarter to one. So, that looks like it’ll work fine.
Perfect. So, one of the things we haven’t discussed is your career as a teacher to undergraduates and a mentor really to graduate students. So, I know with all of your research collaborations and all of your offsite research from Princeton you probably did not have that much opportunity to teach every single semester. But sort of over the course of your career, what have been some of your favorite courses to teach undergraduates in physics?
Until I became chair the rule was that faculty teach every semester, or if one had to be away it was possible to teach double one semester to make up for the semester when you’re not around. So until 1991 I taught full time. It’s wonderful to think back on students who have gone on to illustrious careers in physics, such as Gary Horowitz, Bob Cousins, Deborah Jin, Bruce Jordan, Katherine Freese, Dan Akerib, Rob McPherson, Mark Convery to name a few, and of course our current president Chris Eisgruber – a physics major and Rhodes Scholar – what a pleasure he was. But I still think the best courses to teach are the accelerated courses for the freshman. There’s nothing like it. These kids are just coming out of high school and raring to go. My god, Eric Lander was one of them! But one has to be careful, because they were all top in their high schools by quite a lot, and expected to be number one at Princeton too, only to be shocked when they see how good their fellow students were. We used to put students into the advanced course right away, but the pressures of this course and all the other things freshman have to adjust to led to a lot of “casualties.” To mitigate this problem the main fall freshman course was restructured to include “advanced sections” for students with strong backgrounds, which allowed anyone who was overwhelmed to change to a regular section without losing face or getting a bad grade – a smooth transition so to speak.
When I was a new assistant professor, the department instituted a three-term sequence with the same professor, beginning in the spring semester of freshman year: introduction to electromagnetism, advanced mechanics, and finally advanced electromagnetism. Electromagnetism is so much the heart of physics theoretically and experimentally that the introductory course had to be special. We were blessed to have a famous textbook, Electricity and Magnetism by Edward Purcell, who won the 1952 Nobel Prize for his discovery of Nuclear Magnetic Resonance.
The first teacher was John Wheeler, who was so charismatic he could convince everybody he/she was the next Einstein. Johnny was a pied piper of physics if there ever was one! When Murph Goldberger, the chair at the time, asked me to be next, I realized I had an impossible act to follow – John Wheeler! Only a fool would even think of trying to emulate him. Instead, I would teach them mathematics as well as physics to show them that much of the mathematics required was invented for physics, including differential equations, vector analysis, and matrix algebra. The goal of the first semester was to wind up with Maxwell’s equations and special relativity – not common for a freshman course, but we figured these kids could do it.
At this time Princeton had a “reading period,” a month in January where students did term papers in humanities courses, studied for examinations, and in some physics courses took an extra two weeks of lectures. The latter became extremely unpopular, so we dropped it and just didn’t teach in January. However, to allow the students to decide they wanted to drink from the “fire hose” of the accelerated sequence I instituted a two-week primer course in relativity as a prerequisite. And so, we would spend two weeks teaching them the basics of special relativity: Lorentz transformations, 4-vectors, length contraction, time dilation, E=mc2 and so on. This prepared them nicely for Purcell’s approach to magnetism as a relativistic effect of electricity and gave them “real” experience to help them decide whether the accelerated sequence was for them, or whether the regular courses would be a better match. There were typically about 60 students in this course, so I was given two helpers to divide the work. I would give the lectures; one instructor handled the labs, and the other ran the problem sessions. The students loved it! In their evaluations they gave us grades of 4.7 - 4.8 out of 5 (before grade inflation!). The following year I had the same group for the sophomore courses – a somewhat old-fashioned curriculum covering mechanics and electromagnetism at an advanced level. The mechanics course featured the Lagrangian formalism, rotational dynamics, calculus of variations, fluids, scattering, etc.
The electromagnetism course emphasized radiation: dipoles, Cherenkov radiation, wave optics. I was leading an experiment at Brookhaven laboratory at the time, and to make the course more interesting decided to teach them the relativistic mechanics needed to analyse the actual particle-physics data as we were gathering it. Again, to expose them further to real physics research I arranged for a guest lecture every three weeks or so from one of our famous experimentalists, including Robert Dicke, Dave Wilkinson and Val Fitch. In these ways the students could experience the cutting edge of physics. Only later did I realize how brilliant a group of students I had been given as one figure of merit, six of them won Sloan Fellowships, an unprecedented number. These were true space cadets -- and as icing on the cake I later had the pleasure of teaching a couple of their children.
Other subjects I enjoyed teaching were thermal physics, statistical mechanics and graduate courses in particle physics. One year I created a graduate course in experimental methods for particle physicists, which attracted about 20 super-keen students. When Val Fitch found out about it he said “I can’t understand why anyone shows up for that course,” but they must be getting a lot out of it to come so faithfully.” Val then made a most helpful suggestion: “you should make sure they’re up to date on all this new QCD stuff and the Standard Model,” which was breaking news, especially at Princeton because David Gross and his student Frank Wilczek had recently discovered Asymptotic Freedom in strong interactions (for which they won the 2005 Nobel Prize)..
Serendipitously Frank, now an assistant professor, came to all the classes to learn the experimental side of things. Before I could ask him, one day a student piped up to say “Frank, can you tell us what QCD is?” And so, we got a couple of fabulous lectures from Wilczek explaining QCD to us. Fitch was speechless.
This group was really engaged and were delighted when I suggested that we visit Brookhaven to see the experiments there. My colleague Doug Jensen and I rounded up a couple of vans and drove them out there to spend the day touring several of the most interesting experiments. All in all, this course was the most fun I had as a teacher.
Stew, on the graduate side, looking back to your own experiences and the extraordinary opportunities you got working in particle physics during such a time of fundamental discovery, right, have you felt confident over the decades that you’ve been able to hand graduate students problems that were equally promising? Both in terms of advancing the science and in terms of being helpful as they launched their careers.
I think so. My last student, whom I shared with Jim Olsen, won the Tanaka Prize from the APS for the best PhD thesis in experimental HEP. Altogether I had 6 students in BaBar, all of whom were able to work on important problems in spite of the large scale of the experiment –at the time the largest in the world. Looking back, their experience wasn’t too different from that of students in my previous experiments at Brookhaven and Fermilab, which were perfect for grad students. Even though the groups were bigger in BaBar, the actual work was carried out by small subgroups interested in particular physics topics. So, the mode in which people worked day-to-day wasn’t that different.
During preparation of the experiment the main activities were carried out in groups working on detector systems, computing, and preparing for physics analysis. However, once the experiment was running and taking data people reassembled to pursue the physics topics in which they were most interested. For a focused experiment like E787, there were only three or four main interests. Of course, one was the search for and discovery of K+ → π+ v v, but our students were also able to make much more sensitive measurements of other rare and interesting K decays. Examples included testing the GIM mechanism that suppressed strangeness-changing neutral currents by measuring K+ → π+ μ+ μ- and testing the weak interaction mechanism via the radiative decays K+ → μ+ vγ and K+ → e+ vγ. However, not long after I left for BaBar we decided not to encourage more students to work on E787 because its operations and analysis, though still essential, now consisted of processing more data – walking on well-trodden ground with little opportunity for something new.
In the early years of BaBar, opportunities for students abounded, and I made sure they gained experience in detector design and R&D before moving to physics analysis. Fortunately, Princeton was leading the drift chamber and muon upgrade projects, so they got lots of real first-hand, hand dirty experience. I sent one of them to the factory in Italy where they’re building the limited streamer tubes for the muon upgrade, and they were at TRIUMF for the stringing of the drift chamber, or in Princeton designing electronics. Once the detectors were working they would all join a BaBar physics working group for their thesis analysis that included physicists and students from various institutes.
Once the muon upgrade was finished, Jim and I did not take on any new students in BaBar. I was tied up full time as Dean and Jim had moved over to the CMS experiment at CERN, where we had a very strong group led by Dan Marlow and Chris Tully. It’s now enhanced by Jim and a super assistant professor, Isobel Ojalvo. Mainly thanks to Dan, Princeton is deeply involved in the upgrade of the CMS silicon detector, a new technology for measuring luminosity at high intensities. The students are getting a strong technical background, but for their analysis they’re mostly having to search the existing data set for yet-unobserved rare processes or to improve the precision of earlier measurements. This “analysis-only” thesis work used to be looked down on, but not anymore, because developing the tools and ingenious methods required to perform ground-breaking analyses has become first-class research in its own right. I’m the first to admit that I simply couldn’t do it -- there’s no way, no way! I admit to being completely ignorant when it comes to modern-day computing.
I can understand what is going on at a block level, but as to how the new analysis methods are deployed in detail, forget it. It’s out of my league. When Jim Olsen entered CMS it had already been running for several years, but his BaBar experience with B particles was unique, and allowed him to contribute almost immediately. He was able to produce the best methods for identifying B particles produced in collisions and using them as signals, or as tags for interesting physics, because they’re heavy particles and hence are favored in decays of the Higgs boson. The lion’s share of searches for new particles now must include decay modes into B’s. That was Jim’s first contribution to BaBar, and he followed it with an important analysis and paper on the Higgs and its decays to B’s.
Stew, a really broad question. One of the hallmarks of your career has been the interplay between advances in theory and advances in experimentation. So, if you just survey all of the experiments you’ve been on, all of the collaborations you’ve been a part of, when has the experimentation driven the theory and when has the theory driven the experimentation? And what might we glean from that sort of broad view of physics of the past 60 years?
Okay. As HEP developed after WWII and into the 60’s experiments were driving the theory by discovering new particles, and posing puzzles not predicted by the theory of the time, symmetry violations in the weak interactions being prime examples. Perhaps CP violation is the ultimate one, but ordinary nuclear beta decay led the way with the 1956 discovery of parity and charge-conjugation violations, and the V-A theory that explained them. However, there was still no Standard Model for the strong interaction or electroweak unification, and a “zoo” of ~ 200 baryons and mesons found during the late ‘50s and early ‘60s made it pretty clear there had to be a more-fundamental underlying structure. This data led Gell-Mann, Cabibbo and others to propose SU(3) symmetry as an important way to relate the animals in the zoo to each other, and then to the enormous simplification of the quark model, in which all baryons and mesons were composites of three elementary constituents: the up, down and strange quarks. The next breakthrough came in 1968 when deep inelastic electron scattering experiments at SLAC indicated that the proton was composed of actual pointlike constituents, similar to Rutherford’s discovery of a pointlike atomic nucleus. Initially a great surprise, this work inspired theorists to invent a new theory of the strong interactions, QCD, with fundamental foundations comparable to those of QED.
The spotlight shifted again in1974 when experiments at Brookhaven and SLAC led by Sam Ting and Burt Richter discovered the charmed quark. Strange, unexplained features of recent experiments had given strong hints that something new was lying around, most notably the enormous suppression of K decay modes involving strangeness-changing neutral currents; e.g., KL0 → μ+ μ+ and K+ → π+ v v. Glashow, Iliopoulos and Maiani had shown that a 4th quark c could interfere with u, d and s to suppress these decays. But I don’t think anybody predicted anything as explicit as what was found by Richter and Ting in what became known as the November revolution --It’s still called that! This was a transformational discovery at the level of deep inelastic scattering or the two-neutrino experiment of Lederman et al., where you had to completely change your way of thinking. Another great example of progress coming in steps is the V-A theory of the weak interaction, which explained parity violation and predicted that CP would not be violated -- it said CP is sacred -- as good as gold. But that triggered people to test it anyway and the rest is history.
Sam Ting’s discovery of the J particle was another instance of “following one’s nose:” Lederman and company had done a very rough experiment in 1969 to search for muon pairs and found an unexplained broad shoulder in the μ-pair mass distribution extending out to ~4 GeV. Some people thought it was background, others felt it might be something new, but “nobody” thought it would be a narrow resonance. In fact, a proposal at CERN to explore this region further was rejected because it was only sensitive to narrow μ-pair states, and of course there would not be any! And nobody turns Sam Ting down!
Sam really got it done. A bolt from the blue, but it didn’t take long to realize that the J/ψ was a cc meson, announcing the very c quark that would account for the suppression of flavor-changing neutral currents. The remaining particles in the Standard Model – b and t quarks, the τ lepton and its neutrino, the W and Z bosons, the gluons -- continued to dodge discovery for a while but were all sufficiently well known by the turn of the millennium to demonstrate electroweak unification, provide strong indirect evidence for the CKM model for CP violation, and give a precise prediction for the mass of the Higgs boson.
The precise measurements of CP violation in B decays in 2001 and the discovery of the Higgs in 2012 in a way tied up all the bows, but puzzles and questions remain: why is the t quark so heavy? Can we explain the hierarchy of particle masses? Why hasn’t Supersymmetry revealed itself as a path toward a unified theory and manifestation of dark matter? Why is the Standard Model so successful, and yet so unyielding to evidence of new physics? And so on.
Looking ahead, the future of accelerator-based physics is cloudier (and much more expensive!) than it’s ever been. In the past, experimental knowledge gained at the latest accelerators provided the theoretical insight needed to set the scale of the next step(s) along the energy frontier. It’s amusing to note that the most exciting discoveries at new facilities were not those used to justify the effort and expense to build them (until the LHC). For example, the Berkeley Bevatron was built to find the antiproton, but ended up discovering a huge set of new mesons and baryons. The Brookhaven AGS was built for the ultimate comprehensive study of these particles, but is remembered for two neutrinos, the Ω- baryon, CP violation, and charm. Then there were the colliding beam accelerators. SPEAR was built to study the vector dominance model, but instead found charm and the τ lepton. Fermilab was designed to study deep inelastic muon and neutrino scattering and hadronic physics, but its main glory came with finding the 5th and 6th quarks, the b and the t. Along the way three major colliders, PEP, PETRA and TRISTAN colliders hoped to find the t quark but came up empty. Instead, PEP and TRISTAN were reconfigured into the “B factories,” which found their target by showing the CP violation observed by Fitch and Cronin to be a feature of the Standard Model, as did the LHC, at least in part, by finding the Higgs boson. However, the LHC has unfortunately found nothing else earthshaking, like dark matter, supersymmetry, dark photons, etc.
Worse still, there are no hints to set the scale or roadmap for the next machines. Nature is cagy!
Stew, on that point, last question looking forward. It’s a simple question, but it may be a complex response. Either for the collaborations that you’re still a part of or just more broadly surveying the field, what excites you the most as you look to the future?
Well, I can tell you what worries me. My friend Jim Virdee, founder of CMS and now Sir Jim Virdee, continues to feel that new insight will emerge in the upcoming running of CMS and ATLAS, which should achieve a factor ~100 improvement in sensitivity via increased luminosity, detector upgrades, and improved analysis methods. Even so, these improvements present formidable challenges. It’s amazing to think they’ll be able to do as well as they’ve done, but the Supercollider (SSC) with its 4-times higher energy would’ve made things much, much easier because at the higher energy, the fraction of cross-section that’s interesting was at least a factor 10 higher than at the LHC. We are so fortunate the Higgs was so light – had it been 200 GeV or so CERN wouldn’t have found it and the SSC wasn’t there -- what might have been!
So, anyway, CERN really lucked out. But the question now is: is there a desert above the Higgs mass, and if so, will the upgraded LHC experiments be able to spot any “oases?” Will any new physics show up? I said to Jim Virdee “you know, even 100 times more precise searches for things that have not been found are going to produce at most a few events in the best of worlds. It’s going to take 20 years, and if backgrounds emerge the sensitivity will increase more slowly. Jim’s argument is, and he knows the situation an awful lot better than I do, that this data set will be unique, and there are a lot of things we don’t know now that could be revealed later, such as information from cosmological experiments; and from other developments that could point the way to hidden jewels in the LHC data -- needles in the haystack. That’s part of what’s driving the push, but with COVID and the challenges to producing the upgrades, it’s going to be a dragged-out process.
To me dark matter is really quite exciting. The longtime program of ever-bigger experiments searching for WIMPS (weak interacting massive particles) with noble liquid or silicon-crystal detectors reminds me of the searches for proton decay a few decades ago. The first experiments were hugely exciting because nobody had ever searched before with anything close to their sensitivity. They quickly set a lower limit on the proton lifetime of ~1030 years, and subsequent experiments with a 50,000 ton water-filled detector reached ~1034 years. At this point things ground to a halt, as the detector needed to improve even by a factor 10 is out of the question. Furthermore, there’s no “target” lifetime to motivate an improvement factor of only 10.
Turning to WIMPS in the few-GeV mass range where they are thought to be, or maybe down to 100 MeV, the detectors are now up to several-tons of Xenon, only one order-of-magnitude larger than those of the previous generation, so the likelihood of making a conclusive discovery is not very high but can’t be ruled out. But at best they’ll only see three or four events, with a signal that is not sufficiently unambiguous to draw a definitive conclusion. It’s just a pulse unaccounted for by background estimates. It's a bit like our rare K decay experiment E 787. We found a few events and used the data and simulations to prove they weren’t backgrounds, and then increased our data sample sufficiently to build an airtight case by showing that the characteristics of the events were consistent with a signal.
Similarly with dark matter searches. Even if the most sensitive experiments, Xenon NT and/or LZ, find a few events, it will require much more sensitive follow-on experiments to show their properties are consistent with those of dark matter. So, if you’re going the WIMP route, you’re going to need a third-generation experiment and I don’t think a factor 10 is good enough. This means new detectors with hundreds of tons of Xenon or argon. The Canadians are dreaming of such an experiment at SNOLAB, that may eventually materialize because of the support of Nobelist Art McDonald, without which it would not have a chance. In any case it would be 20-30 years before it could bear fruit.
The liquid argon (LAr) situation is worth discussing. Ordinary atmospheric argon contains small amounts of Ar39, a radioactive isotope that makes multi-ton detectors impossible (it’s the same substance that causes radon in people’s basements). Amazingly, about 20 years ago Princeton scientists discovered an oil well in Colorado where the extracted gases have been deep underground so long that the radioactive isotope has died out completely -- at least 1,000 times less radioactive than surface argon. Tons of it are about to be mined for a 20-ton experiment in Italy. Once extracted it will be sent to Sardinia for further purification and then quickly put underground in the Gran Sasso lab before being reactivated by cosmic rays. An extraction system is under construction that will make it imaginable to marshal 1,000 tons of liquid argon 10 years from now, and then a new experiment would have to run for another 10 years or so. To me this resembles the proton decay situation – to make any useful progress is a huge undertaking with diminishing returns.
Fortunately, WIMPS are not the only game in town! If one asks the young theorists, one finds a new burning interest in other manifestations of dark matter that can be searched for with much smaller, quicker experiments. There are new conferences, new excitement, new people, new students – it’s a brand-new field. In fact, that’s the one area that really gives me enthusiasm in particle physics. Can we find the dark matter? Maybe somebody will hit the jackpot, but we won’t know until they’ve done it. But if I look around and think, what can we do, this direction opens new territory and seems more promising than spending decades and vast resources in directions that have already been well mined.
Well, Stew, on that note, it has been an absolute blast spending this time with you. Thank you so much for spending this time and sharing your insights.