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In footnotes or endnotes please cite AIP interviews like this:
Interview of Junko Shigemitsu by David Zierler on April 2, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
In this interview, Junko Shigemitsu, Professor Emerita in the Department of Physics at the Ohio State University, surveys the field of lattice gauge theory over the course of her career, and she recounts her childhood moving around the world because her father was a diplomat for Japan’s foreign ministry. She explains the circumstances that led her family back to Japan, and her decision to pursue a degree in physics at Sophia University in Tokyo. Shigemitsu discusses her interest in attending Cornell for graduate school, where she studied under the direction John Kogut. She discusses Ken Wilson’s revolutionary work on renormalization, and her thesis work on QCD. Shigemitsu describes her postdoctoral work at the Institute for Advanced Study at a time when lattice gauge theory was beginning to mature, and she discusses her subsequent postdoctoral position at Brown. She explains that opportunities that led to her faculty position at Ohio State and her subsequent research on QCD at non-zero temperatures. Shigemitsu discusses the international HPQCD collaboration and more recent advances in understanding subatomic particles in partnership with SLAC and KEK in Japan. She places the greatest excitement in finding physics beyond the Standard Model in the period starting in 2009, and she explains the increasing utility of computers as their power has grown over the decades. At the end of the interview, Shigemitsu conveys her excitement that the field will yield new discoveries, perhaps including new physics, and that quantum computing will likely be central to these prospects.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is April 2nd, 2021. I'm delighted to be here with Professor Junko Shigemitsu. Junko, it's great to see you. Thank you so much for joining me.
Thank you for taking the time to interview me.
Junko, to start, would you please tell me your title and institutional affiliation?
I am professor emerita in the physics department at the Ohio State University.
Junko, when did you go emeritus?
In 2017. Four years ago.
And in what ways in the past four years have you done the kind of physics that you've wanted to do where you didn't have any concerns about other responsibilities?
At the time I retired, I was still working on several research projects with former postdocs and with long term collaborators at other institutions. So, during the first couple of years after retirement, I focused on completing and writing up those projects. I was then just starting to engage in new activities when the COVID-19 pandemic changed our lives.
On that question, Junko, to what extent did the pandemic affect your research one way or the other? In other words, did it give you some more time to work on some problems you might not otherwise have had to do? Or on the other hand, to what extent do you really rely on in-person contact with people in order to get your most important work done?
Many of my research collaborators have been from outside Ohio State, so I was used to communicating with them via Skype or Zoom. So that did not change much due to the pandemic. But I consider myself lucky that I was retired already when the pandemic hit. I had no teaching or mentoring duties and did not have to scramble to switch from in-class to online lecturing.
Junko, just a snapshot in time right now, what's happening in lattice gauge theory or QCD that's most interesting and exciting? Just generally in the field, what's going on right now?
I should mention that I have been working in lattice gauge theory most of my career. There have been many exciting times during those years. But, for me, the most exhilarating periods of my scientific career have been the first ten years and then the final ten to fifteen years before I retired.
Yes. The first ten years, starting from the early nineteen seventies, constituted the period when the Standard Model of Particle Physics was established. This followed one exciting discovery after the other by experimentalists and the hard work of many theorists. Quantum Field Theory became the language that particle theorists needed to employ to study the Standard Model and work out the implications of this Model. I was lucky that the Standard Model emerged just when I was working as a graduate student at Cornell University, learning what Particle Physics was all about and the technical tools necessary for theoretical work in this area.
Lattice Gauge Theory entered Particle Physics with the seminal paper, “Confinement of Quarks”, by Kenneth G. Wilson, Professor at Cornell, in 1974. It focused on the Strong Interaction part of the Standard Model, called Quantum Chromodynamics or QCD. By formulating QCD on a space-time lattice, the strong interactions could be studied using theoretical tools known from Statistical Mechanics and Condensed Matter Physics. In particular one could go beyond perturbation theory, the approach used by most particle theorists prior to Wilson, to do calculations in Quantum Field Theory. So the emergence of the Standard Model, including QCD, together with new theory methods, such as lattice gauge theory, made for very exciting times.
Let me now fast forward a couple of decades, let's say to soon after entering the 21st century. I think that around that time lattice gauge theory finally matured enough to be able to do precision calculations in QCD. It started to obtain several results that were useful to the broader particle physics community. In particular, lattice gauge theory could provide crucial QCD input needed for precision tests of the Standard Model. Reliable Standard Model predictions together with stringent consistency checks of the Standard Model are very important in the search for “Physics Beyond the Standard Model”, a major goal of the particle physics community in the coming decades. Soon after entering the new century, interactions between particle experimentalists, phenomenologists and lattice gauge theorists increased significantly compared to during the two and a half decades prior to that. The three communities started to work together in planning which quantities to measure and how to reduce total (experimental and theoretical) errors. Many workshops were organized, leading to very fruitful exchange of ideas. I found these developments very rewarding and exhilarating.
It's always an exciting time when theorists and experimentalists want to work with each other.
Yes, and the different communities end up stimulating and motivating each other. I am hopeful that eventually experiments and theory will become so precise we will be able to observe the Standard Model breaking down and discover hints of what a more fundamental theory might look like. Although there are already some tantalizing apparent discrepancies between experiment and Standard Model predictions, errors are still too large to be able to claim that New Physics has definitely been discovered.
It's remarkably stubborn, the success of the Standard Model, isn't it?
Yes, it is. There are hints from astrophysical observations, existence of Dark Matter and Dark Energy or the dominance of Matter over Anti-matter, that our Standard Model is incomplete. But we do not know at what energies New Physics will become visible in accelerator experiments that are pushing the high energy frontier, nor how small errors have to become before the breakdown of the Standard Model manifests itself in high precision experiments. I am still optimistic that eventually we will get beyond the Standard Model, but so far, Nature hasn’t...
(laughs) Well Junko, we'll develop all of these issues as we go through the narrative of your career. But first, let's take it all the way back to the beginning. Let's start first with your parents. Tell me about them and where they're from.
My family is from Japan, and I am also still a Japanese citizen and a permanent resident of the US. My father was a diplomat, and my mother a very successful and enthusiastic diplomat's wife.
Your father worked for the Foreign Ministry?
Yes, for the Japanese State Department.
What years did he serve?
My father entered the Japanese foreign service just before World War II and retired in 1978.
That's an exciting time to enter the Foreign Ministry.
There certainly was a lot going on then. It turns out, both my parents spent the Second World War in Europe, though they were not married yet then.
Where was your father stationed in Europe?
He was in Warsaw when the Germans invaded Poland and WW II began. After Poland, he was stationed in Bulgaria for a couple of years. That is where my parents met for the first time. My mother’s father, my grandfather, was also a diplomat and was stationed in Sofia at that time. From Bulgaria my father was sent to the Japanese Consulate in Vichy, France. And finally, when the Third Reich collapsed, my father, like most Japanese diplomats in formerly German occupied territories, ended up in Switzerland. My mother also spent the last months of WWII in Switzerland.
Did your parents ever talk about their experiences to you? Did you learn about what they went through?
They never spoke very much about those times. But my impression is that as diplomats their lives were more comfortable than those of average Japanese back in Japan during WWII. They were not subjected to all the air raids and food shortages, that citizens not only in Japan but also in many parts of Europe had to endure.
Where did your parents go after the war?
After Japan surrendered, all Japanese diplomats and their families were sent back to Japan. They gathered in Naples from all over Europe and were put on a ship for the long journey through Gibralter, around Africa, across the Indian Ocean, via places like Singapore and finally to Japan.
So your parents were not in Japan when the atomic attacks occurred.
No, they were not. But my father recalls going with the Japanese Ambassador in Switzerland to the head of the International Red Cross in Bern and showing him photographs of the devastation in Hiroshima.
Did your father remain in the foreign service?
Yes, he was young enough that he could remain. Senior people, above a certain rank, such as my grandfather all had to retire.
Did he continue serving abroad, or he was mostly based in Japan after the war?
After the Allied occupation of Japan ended in 1952, my father spent most of his time abroad, with just a couple of stints at the home office in Tokyo.
Junko, where were you born?
In Japan actually.
Yes, in Tokyo. The first five years of my life were spent in Japan. Then, between 1954 and 1957 my father was stationed in London. After that our family spent two years in India and then went back to Japan for two years. From 1961 to 1965 my parents were in Moscow, while my sister and I went to a Boarding School in Vienna.
So obviously, you picked up English early as a five year old?
Yes, that's true.
I detect a slight British accent.
Yes. Living in Columbus, Ohio, I am amused when people sometimes ask me, "Are you from Boston?" Some people cannot imagine anything more British than Boston, as if Great Britain itself did not exist.
Junko, looking back at your childhood, was your education, did it feel very disrupted, that you couldn't be in a place long enough? Or did you adapt pretty well?
I must have adapted quite well. At least I do not recall ever suffering or struggling when we came to a new country or started at a new school. My sister and I just accepted that life was like that. I think children are usually very flexible.
Was it in the boarding school in Vienna that you started to experience math and science classes at a high level?
I don’t really remember how much physics was taught at my Viennese school, but math instruction was quite rigorous and at a high level. Science education was emphasized more in the last two years of high school, which I attended in Romania. My father was stationed in Romania then and there was a German school there, because Romania has a large German minority. It was easy for me to switch to this school because I knew German already from Vienna.
What options did you have for college? Where could you have gone to school?
In many of the European countries passing the high school graduation exam entitles you to study at a university. This exam is called the Abitur in Germany, or baccalaureate in France, and in Austria it is called the Matura. I was able to go back to Vienna and take the Matura there. I then spent one year at the University of Vienna. However, when my father was stationed in Japan again, our entire family decided to go back to Japan together. And I joined the physics department at Sophia University in Tokyo.
Did you know you wanted to study physics before you got to college?
Yes, by the time I finished high school, I knew I wanted to study physics.
And why physics? What was interesting to you about physics as a high school student?
Well, I am not quite sure, but physics seemed to be the most basic among all the scientific fields.
Junko, did you have an appreciation that there was a lot of excellent physics that was happening in Japan at the time? That there were many world-renowned Japanese physicists? Did you know this?
No, not at all. I was woefully ignorant about the state of physics research not only in Japan but also elsewhere in the world.
Tell me about Sophia University. How did you choose that?
Well, you may have heard that entrance exams for Japanese universities are very competitive. People start preparing already at a very young age. My only pre-college experience with the Japanese educational system were two years of elementary school between ages 10 and 12. So, I certainly was not prepared to take any university entrance exams. Sophia University is a small Jesuit university with an international outlook. And they allowed me to just transfer from the University of Vienna.
I gathered from the name it wasn't just a Japanese university.
I was lucky that they took me in. However, the physics department was very new in 1969 and my class was only the second one to graduate from this department. There were further complications since Sophia University, together with most other universities in Japan, was also affected by the Student Activism of the late sixties.
I wanted to ask about that.
Sophia University was in lockdown for half a year before I started there.
So the campus protests that convulsed universities all over the world, they reached Tokyo?
Yes, they happened in Japan too.
What were students protesting at this time?
Some of it was protesting the Vietnam war. Closer to home, the left was against the American-Japanese security treaty and also demanding the return of Okinawa to Japan.
It sounds like the connecting thread here was a critique of American power in Asia?
Yes, that was the fashionable thing for leftist groups, especially among students.
Junko, maybe it's a funny question, but I'm curious if you felt as Japanese as your fellow students, given that you spent so much of your childhood not in Japan?
Yes, it is true. I was very ignorant about Japan. Even though I could speak Japanese, my written Japanese was very bad. Fortunately, I was a physics major, and we did use English textbooks like Schiff and Goldstein, the same as in the US, so that worked out. I should mention that I was already an anomaly in my class since I was the only female student in my year.
Junko, what kind of physics class did you like the most as an undergraduate? What kind of physics did you think you wanted to do for a career?
Well, again, I was woefully ignorant about all the different research areas in physics. I vaguely thought I wanted to do particle physics. First, I wanted to learn quantum field theory properly, because that was only hinted at in our undergraduate curriculum. I applied to various graduate programs mainly in the US. But at that point I really had no idea what graduate school involved and what it would take to become a particle physicist.
Did you know at least if you wanted to focus on theory or experimentation for graduate school?
Yes, I was definitely interested more in theory.
And what kind of advice did you get about graduate school? Did you have professors who specifically said you should go to the United States?
No. As I said, it was a very young physics department, though several professors had spent time abroad as part of their education or as postdocs. The person I spoke to most was a professor who had spent time at Cambridge University in the UK. The US seemed the most welcoming to international students. There was an American Cultural Center close to Sophia University, where one could get information on how to apply to American graduate schools. Also, it was easy to get financial aid. If you were accepted into a program, you were also offered a position as either a graduate research or a teaching assistant, and tuition was waived.
Where did you apply in the United States?
Let me see. I applied to Cornell, Columbia, Harvard, Stanford, Purdue and Rochester.
Why ultimately did you choose Cornell?
I was accepted by Cornell, Columbia, and Rochester. I didn’t know too much about any of these three places, so for me it was basically a toss up between Cornell and Columbia.
In all of your travels, had you been to the United States before?
No. My family had spent so much time in Europe, but neither my parents nor my sister or I had ever lived in the US before I moved there.
Did you know that Ithaca was such a small town? Did you realize you were going to such a small place in the country?
No, I only discovered that after arriving at Cornell and that was a bit of a shock to me. In fact, that was the hardest thing for me to get used to, more than the fact that I was entering graduate school for the first time or that I was in a new country. Until then, I had always lived in large cities like London, Tokyo, Vienna, Moscow. But fortunately I was young then, so I adapted.
Junko, what were your impressions of the physics department once you got settled in?
The physics department was very good, I thought. They looked after their students and the professors were easy to talk to. So that part I was happy with.
Who became your graduate advisor? How did you develop that relationship?
My graduate advisor was John Kogut, who was a young assistant professor at that time. He was very popular with the graduate students and had about 60% I think of the particle theory graduate students working for him. He was a very good mentor.
And what was he working on? What was his specialty?
He was working on renormalization in Quantum Field Theory, on QCD and, of course, on Ken Wilson’s lattice gauge theory.
But this is before 1974. Where is lattice gauge theory when you arrive at Cornell? What's going on before 1974?
By 1974 Ken Wilson had already done much of his important work on the Renormalization Group and on Critical Phenomena. His unique approach to “physics at many scales”, was more fundamental and far reaching than, for instance, the renormalization step one goes through in perturbative Quantum Field Theory. He developed many of his ideas and intuition by studying Condensed Matter systems undergoing phase transitions. Since models in Condensed Matter physics are often spin models on a lattice, when Wilson subsequently started looking for his own way to study QCD, it must have been quite natural for him to formulate QCD, or gauge theories in general, on a lattice.
Was your sense that what Ken Wilson was doing, how revolutionary it was, was that already well-appreciated at that time? Or this was happening, these things were developing right when you came to Cornell?
I think Ken Wilson’s Renormalization Group work was appreciated initially more by the Statistical Mechanics and Condensed Matter Theory communities than by Particle physicists. Wilson was communicating a lot with people like Leo Kadanoff, Michael Fisher and Ben Widom. By 1974, however, leading Particle theorists were also paying attention.
So from your perspective, Junko, as you were learning these things, for you what was so revolutionary about what Ken was doing?
To be honest, when I was first learning Quantum Field Theory as a second year graduate student in1974, I was still too inexperienced to appreciate how much deeper Wilson’s approach was compared to what one reads about in standard textbooks.
Junko, what was the intellectual process for you to develop what would become your thesis research?
Well, my thesis was actually rather a hodge podge of several projects, the common thread being that they all investigated some aspect of QCD. The title of my thesis was “Topics in QCD”, and I essentially pasted together three papers I had written as a graduate student, two on lattice gauge theory and the third using an approach called “The Asymptotically Free Parton Model”.
As we were saying earlier, at the first part of your career and the last part of your career were both really exciting in the ways that theorists and experimentalists interacted. Do you include your thesis research in this? Were experimentalists really important for the things that you were looking at?
Experimentalists were of course, crucial for establishing the Standard Model. They told us what kind of models we should be working on. Critical were the deep-inelastic scattering experiments, discovery of neutral currents, discovery of the charm and bottom quarks, which all occurred before I got my PhD in 1978.
Did people like Gross or Politzer, did they visit Cornell? Did you go to talks?
I do remember David Gross coming to Cornell to give us a seminar. At that time he was pushing the “Instanton” approach to QCD.
What was so important about the discovery of asymptotic freedom to your research? How did it change things for you?
The discovery of asymptotic freedom in NonAbelian Gauge theories, such as QCD, by Gross, Politzer and Wilcek was very important to QCD being acceptedas the theory of strong interactions. Asymptotic freedom could explain the initially unexpectedly weak interactions among the constituents inside a proton, as observed in the deep-inelastic scattering experiments mentioned above. It is also consistent with the idea that the QCD coupling gets very large as one goes to large distances, so large in fact that quarks are permanently “confined” inside a proton. This property of QCD is consistent with the experimental fact that individual quarks have never been seen in isolation. Weak coupling at short distances and large couplings as one goes to large distances, as required by experiment, is the hallmark of an asymptotically free theory.
I should mention that in lattice gauge theory, it is easy to show that quarks are confined at large coupling. In his 1974 paper, Wilson used strong coupling expansions to demonstrate that the potential between a heavy quark and anti-quark grows linearly with the distance between them. QCD formulated on a lattice contains a linear confining potential. In order for lattice QCD to make contact with asymptotically free continuum QCD, it is important that the lattice theory have, using the language of critical phenomena, an ultraviolet fixed point at the origin in coupling constant space. Furthermore, in order that the confining properties of lattice QCD not be lost as one goes to the continuum, lattice spacing going to zero, limit, it is important that no phase transition separates the strong and weak coupling regions of lattice QCD. All calculations to date indicate that lattice QCD meets these requirements.
Junko, looking back, were there any particular advantages or disadvantages to having a more junior faculty member as your advisor?
No, I would not think a professor’s age is the determining factor in whether the person is a successful mentor or not.
Who else was on your thesis committee?
Let me think. Kurt Gottfried and Bob Richardson.
Anything memorable from your oral defense? Any questions that stick out in your memory?
Well, only that it went on for a long time, and not because they were grilling me, but because we got to talking a lot. And so friends who were preparing a celebration had to deal with the ice cream cake melting as they waited for the exam to be over.
(laughs) Did you take the length of the defense to be a good sign or a bad sign while you were in the middle of it?
Well, good in the sense that the discussions were lively. Committee members were interested in my results, so I think that was a good sign.
After you defended, what did you want to do?
Well, I don't know, sleep I suppose.
Were you recruited for postdocs? Did you apply yourself?
Yes, I applied to several places, and eventually ended up at the Institute for Advanced Study in Princeton for my first postdoc.
Where did you apply? Where were you interested in working?
Just like most other people in the postdoctoral market, I applied indiscriminately to many places without considering whether they had an opening or not or what kind of expertise they might be looking for.
What were your impressions of the Institute when you first arrived?
I liked the intellectual atmosphere there. Also the grounds at the Institute were beautiful and serene. It felt a little bit like being at the estate of a rich lord in a 19th century novel. In addition, it helped that Princeton was just an hour’s train trip away from New York City. It was easy to visit the City just for the day. So, yes I really enjoyed my two years at the Institute.
Who were some of the luminaries at the Institute that you may have been excited to work with or to get to know?
Roger Dashen and Steve Adler were the two permanent Particle Theorists who were looking after the postdocs. There were other famous people as well……
Dyson? Freeman Dyson? Is that who you're thinking of?
Yes, Freeman Dyson was there too.
Did you enjoy being in an intellectual environment where it was not just physicists?
Yes, that was part of the charm.
Junko, did you take the postdoc as an opportunity to improve and expand upon your thesis research, or to take on new projects?
I was eager to engage in new projects though still related to lattice QCD. I ended up working mainly with other young postdocs at the Institute.
And what does that mean, circa 1978, "how far lattice gauge theory would go"? Where could it go?
Lattice gauge theory was still very young and only a few people were pursuing it around 1978. So, it was not clear at all how far this approach could go. At that time, most theorists’ experience with Quantum Field Theory came from Quantum Electrodynamics, QED, which is the field theory associated with electromagnetic interactions. Once the Standard Model was written down, particle physicists realized that other interactions, such as the Weak and the Strong interactions, were also described by gauge theories, actually gauge theories more complicated than QED. So there was a big push to better understand gauge theories in general and develop tools to analyze them. Especially for the strong interactions, for QCD, one needed methods that did not rely on perturbation theory. Lattice gauge theory was one of the new approaches that promised to be able to do just that.
There was rapid progress between 1978 and the early eighties. In 1980 Michael Creutz from Brookhaven National Laboratory published the first paper on studies of Ken Wilson’s formulation of QCD on a lattice via numerical methods on a computer. Creutz was able to get away from the strong coupling region of lattice QCD and find behavior consistent with asymptotic freedom at weak coupling. Ever since then, lattice gauge theorists have turned to numerical simulations on the fastest computers available as their primary calculational tool. Ken Wilson, right from the beginning, always stressed that computers would be very important for the success of lattice gauge theory.
And in the world of experiment, was anything going on during your years as a postdoc that were interesting or compelling to you?
A lot was going on in the experimental world, solidifying the Standard Model.
Did you ever spend time at SLAC?
Yes, during my last year as a graduate student. John Kogut, my thesis advisor, was on sabbatical and spent one semester at SLAC. And he took two of his students with him, Steve Shenker and me. Yes, that was nice.
Were you on the job market during your time at the Institute? Or did you want specifically to do another postdoc?
I applied only for postdoctoral positions from the Institute, not for any faculty positions.
And is that specifically because the job market was not good, or you wanted more time as a postdoc?
Well, for both those reasons.
How did the opportunity at Brown come about for you?
Brown University was one of the places I applied to that made me an offer. I knew Antal Jevicki there, since we overlapped as postdocs at the Institute. Brown was also attractive to me since it had a strong condensed matter theory group that was working on Ken Wilsons’s renormalization group ideas.
Was Brown productive? Were you productive during your years at Brown?
Yes, I suppose so.
What were you working on during those years? What were your major projects?
I was working on several different types of lattice gauge theories in order to get deeper insight into these models. The list includes Z(N) gauge theories with Z(N) matter fields, SU(N) gauge theories for general N, gauge theories at nonzero temperatures and chiral symmetry breaking for quarks in different representations of SU(3). All this was quite new for the lattice community.
And when you say, "everything was new," does that mean that even from the time when you were doing your thesis research? That it just still remains new? Or that the field is expanding, and in those expanding areas, that's where the new stuff is?
Yes, the field was expanding.
Junko, at this point, was lattice gauge theory, QCD, was there a sociology of the community in physics that was working on these things? In other words, was the field mature enough at this point that there was a lattice gauge theory and a QCD community?
The lattice community was still quite small. We did, however, already have the first workshop on lattice gauge theory in the Summer of 1980 in Santa Barbara.
And at this point, you were ready after Brown to go on the job market?
Yes, in the Fall of 1981 I started to apply for faculty positions.
And were the faculty positions specifically in lattice gauge theory, QCD?
No, physics departments usually advertised positions in particle theory without specifying any particular sub area within theoretical particle physics.
What jobs were available to you that year, when you started to look?
Among research universities, there were, I think, only about three places searching for a particle theorist that year, Berkeley, Ohio State and Rochester.
How specifically did the opportunity in Ohio come about for you?
I did not know much about Ohio State nor about Columbus before I applied. But the rules of the job market game dictated that one apply everywhere that has a particle theory opening and then hope for the best. When Ohio State asked me to come for an interview, I was very impressed by how energetic and enthusiastic the faculty there were about trying to build up physics research at Ohio State.
Was it your sense that the department was looking to grow and gain in stature?
Yes certainly. I believe in the early eighties the Ohio State physics department made a conscious decision to put more emphasis on research, not just in particle physics, but in condensed matter and other areas as well. They hired a lot of assistant professors around the time I joined them.
Junko, by the time you took the faculty position, what research projects were you working on at that point?
The largest projects were in collaboration with John Kogut, who had moved to the University of Illinois by then, with Michael Stone and Bill Wyld, both also at Illinois, and with Steve Shenker at Chicago. We were studying QCD at nonzero temperatures, with focus on the deconfining and chiral symmetry restoring phase transitions. For these investigations we, together with most of the rest of the lattice community, had already transitioned to using numerical methods on computers. So, when I first started at Ohio State, I was initially busy learning to write computer codes and to work with super computers.
Junko, when did you start working with Peter Lepage?
We started working together on lattice gauge theory around 1992.
What kinds of courses did you first teach when you got to Ohio?
Most faculty members teach a broad range of courses, alternating between undergraduate and graduate classes. So I too went through the large introductory courses, junior and senior level courses for physics majors, and then also a variety of graduate courses.
Did you take on graduate students right away?
Throughout my career, I actually had very few graduate students. I worked more with postdocs than with graduate students. At Ohio State, there were not that many particle theory graduate students to begin with. And those who did express interest in particle physics usually preferred working on more theoretical topics like supersymmetry or string theory. Lattice gauge theory had the reputation of involving a lot of drudgery, such as debugging code or analyzing numerical data, “too much hard work” as a student once told me.
Why does it have that reputation? Why would people think that it has too much work?
It is true that lattice gauge theory projects tend to take more time to complete than with work in other areas of particle theory. In that sense, lattice gauge theory sits somewhere between experimental and typical theoretical work.
Junko, what were the origins of the international HPQCD collaboration? How did that start?
Peter Lepage had been pursuing the Nonrelativistic approach to studies of heavy quarks, the bottom and charm quarks, in both continuum and lattice QCD. In the early nineties he formed a collaboration of lattice gauge theorists to embark on state-of-the- art numerical simulations of systems of bottom quarks and antiquarks. In the summer of 1992, my then postdoc John Sloan and I visited Cornell as part of this collaboration and we all started working on developing the computer codes for this project.
And this was the early 1990s?
Yes. We did not call ourselves the HPQCD collaboration yet. That happened only around 2002. If I remember correctly, I think we were the NRQCD (Nonrelativistic QCD) collaboration in the early nineties and then went through a couple of other names during the nineties.
Junko, during its early years, the collaboration of course was small. How big, ultimately, did it get? How many people were involved and how many countries were involved?
We were never a large collaboration compared to most other lattice collaborations in the US, Europe or Japan. This remained true even as we later significantly expanded the systems and topics we studied beyond the heavy quark systems we started out with. The core senior members were Peter Lepage at Cornell, Christine Davies at Glasgow, myself at Ohio State, joined from time to time by Howard Trottier at Simon Fraser and/or Ron Horgan at Cambridge. So 3 to 5 institutions in 2 to 3 countries. And each institution had a couple of lattice postdocs and students. Two Ohio State postdocs became faculty and continued working with HPQCD. Matt Wingate ended up at Cambridge and Chris Bouchard at Glasgow.
Was the aim of the collaboration always to understand better the properties of subatomic particles? Was that always the fundamental mission?
Understanding the Universe in terms of its most fundamental building blocks, the elementary particles, has always been the goal of particle physics. Our collaboration started initially by calculating the masses of bound states involving heavy quarks and antiquarks. We tested QCD by comparing our numbers with experiment wherever such data was available. Or we ended up making predictions for states that experimentalists could be looking for. By the mid nineties we also had results for the strength of the QCD coupling and for the bottom and charm quark masses.
We then expanded our focus to include systems with both heavy and light quarks. There was considerable interest in the particle physics community to study “B meson” and “D meson” systems, B mesons being boundstates of a bottom quark with light (“up”, “down”, or “strange”) quarks and D mesons boundstates of charm quarks with light quarks. It was known that important properties of the Standard Model could be extracted by observing decays of B and D mesons. So-called “B factories” were designed, funded and constructed and several experimental collaborations were formed, the BaBar collaboration in the US at SLAC and the Belle collaboration in Japan at KEK. Another experiment, the CLEO-c collaboration, was set up at Cornell to study D mesons.
Lattice gauge theory started to play an important role in analyzing experimental data coming from BaBar, Belle and CLEO-c. Lattice calculations provided crucial QCD input necessary to extract physics from experimental measurements. Initially, both experiments and lattice QCD calculations had large errors. However, soon the race was on as to which experimental or lattice collaboration could come up with the smallest errors. It was around this time that many workshops involving experimentalists, lattice gauge theorists and continuum phenomenologists were organized. In fact, at least in the US, funding agencies demanded that these communities communicate and work with each other.
Chronologically, when did that happen when your colleagues in experimental high energy physics recognized that the work of the collaboration was really going to be helpful in the way that they did their measurements?
I think it started around 2000. Needless to say, we, the HPQCD collaboration, were not the only lattice collaboration working on B and D physics. There were several others in the US, Europe and Japan. And on the experimental side, once the Large Hadron Collider, LHC, started running at CERN in Switzerland in 2009, the LHCb collaboration became a major player in B and D physics.
And did this coincide as you were saying before, when there was most excitement about finding new physics beyond the Standard Model?
Yes, by that time the emphasis had already shifted from just verifying the Standard Model to looking beyond it.
What were the theoretical assumptions that made you and your colleagues think that new physics beyond the Standard Model could be found, and what was the experimental data that demonstrated actually not, we're still in the Standard Model? Please describe how these things came all together.
Any discrepancy between experiment and Standard Model predictions would signal the need for New Physics. The US Department of Energy, the DOE, used to divide its program in terms of “Frontiers.” The “Energy Frontier” focused on going to higher and higher energies at accelerators. The goal was to investigate whether a new type of particle or particles would suddenly appear above a certain energy threshold, that are not part of the Standard Model. Such direct observation of new physics has not happened yet.
Although several Beyond the Standard Model theories exist, none can really accurately tell us where the so-called new physics energy threshold lies. Would a factor of two increase in accelerator energy get us there? Or is it a factor of ten or a hundred? I don’t think we really know.
Another approach to searching for new physics is via the “Precision Frontier”. Here one focusses on the possibility that new physics will manifest itself first as small corrections to the Standard Model, which leads to the obsession with reducing errors. For instance, if new physics is a 1% effect we will not see it unless the combined experimental and Standard Model theoretical errors are under 1%. It turns out that B and D meson systems, mentioned above, contain many processes where new physics could show up. This is the reason why so many lattice groups, including HPQCD, and experimental collaborations, BaBar, Belle, CLEO-c and LHCb, are spending so much time on them. Another important quantity where people hope to see new physics and where extreme high precision is required is the “muon anomalous magnetic moment” or g-2(muon).
In spite of significant progress in the past decade in reducing errors, unfortunately we still have not seen unambiguous signs of any discrepancy between the Standard Model and experiment.
Junko, because so many physicists are working from so many different areas to find new physics beyond the Standard Model, from your perspective, the fact that lattice QCD, lattice gauge theory QCD, have not, what might that tell us more broadly about both the Standard Model and the capacity for physics in its current form to be trapped in the Standard Model, as it were?
To be honest, we really do not know how long it will take to find new physics. As I’ve already mentioned above, we do not know how high in energy accelerators need to go, or how precise experiments and Standard Model predictions have to become, before new physics manifests itself directly in particle physics experiments.
Well, Junko, now that we've worked up to the present, for the last part of our discussion, I'd like to ask some broadly-retrospective questions about your career and then we'll end with a question looking to the future. So one thing we haven't talked about yet, which I'd love to hear your perspective on, is the role of computers, and specifically the exponentially increased computational power that very much coincided over the course, the chronology of your career. So at what point did computers become relevant for your research, and when did computers become powerful enough where they might really drive where the research was going and how the data might be interpreted? I wonder if you could speak broadly to these things.
Computers became important for lattice gauge theory almost from the start. By the beginning of the eighties, we were all looking around to secure access to cycles on fast machines. The most powerful computers were then at national labs, with many of them involved in classified work. Universities had at most a computer center with slower mainframe machines. The situation changed quite rapidly in the eighties, when funding agencies started supporting computing for University based research in a wide range of disciplines. The NSF created supercomputer centers at Illinois, Cornell, Pittsburgh and San Diego. The DOE opened up the National Energy Research Scientific Computing Center, NERSC, located initially at Lawrence Livermore National Lab, to academics throughout the US. The lattice community rapidly got the reputation as having insatiable appetite for computing cycles.
On the other hand they were welcomed as “friendly users” when funding agencies were installing new machines at national labs or academic computer centers. They could be counted on to quickly be ready with codes suited for the new machines and test the new hardware to its full capacity. At Ohio State, a faculty driven initiative (that I was also a part of) succeeded in convincing first the University president and then the State legislators to fund the Ohio Supercomputer Center, OSC, to support research and student training throughout the state of Ohio. OSC began operations in 1987 and is still going strong today. Throughout the late eighties and the nineties, the Ohio State based lattice group did most of its numerical work at NERSC and at OSC.
By the end of the nineties the US lattice community realized that the next big step in making lattice calculations more accurate would require supercomputers dedicated just to lattice gauge theory. We also felt that we had a strong case to argue that such lattice QCD precision calculations would fit in and contribute significantly to the success of the broader particle physics program, including the search for beyond the Standard Model physics. DOE agreed with this assessment. They stipulated, however, that there be one designated contact person who was to speak for all US lattice gauge theorists seeking computing resources. With that in mind, the “USQCD Collaboration” was formed in 1999, and it is essentially a collaboration of lattice collaborations. Large scale clusters optimized for lattice calculations were installed at Fermilab, Jefferson Lab and Brookhaven National Lab dedicated to lattice gauge theory. USQCD has an Executive committee, whose chair becomes the contact person with DOE. There is also a Scientific Program committee that reviews proposals and allocates computer time to individual lattice collaborations in the US. The HPQCD collaboration, of course, joined USQCD and the US based HPQCD members started using the Fermilab clusters around 2004.
Finally, I should mention that it was not only the huge increase in computing power that lead to accurate lattice results. The hard work over decades in algorithm improvements and development of better ways to transcribe continuum QCD onto a lattice played important roles as well. One of HPQCD’s contribution was the introduction of the HISQ or “Highly Improved Staggered Quark” action in 2007. Several lattice groups are now employing the HISQ action in their simulations. This way of putting quarks on a lattice has significantly smaller discretization errors than older actions.
Junko, last question looking to the future. What might change and what surprises might there be for lattice gauge theory and QCD that would make the kind of excitement that you felt at various stages in your career feasible?
Well, surprises are what scientists welcome. Unexpected discoveries will necessitate new ideas, which will eventually lead to deeper understanding of nature. So I, together with all other particle physicists, am eagerly awaiting more experimental data.
Do you think that the current experiments running will yield that data, you are waiting for, or do you think we’ll need new experiments, new collaborations, to yield that data?
I don’t think anybody knows for certain what will be seen at particle accelerators in the next decades. Recently, surprising discoveries have occurred, I think, more in astrophysics, acceleration of the expansion of the universe, dark energy, the high rate of blackhole mergers in the universe leading to emission of gravitational waves and so forth. Of course, these discoveries are of great interest to particle physicists as well. Our theories in particle physics have to be consistent also with all astrophysical data.
Is it possible that quantum computing will offer a breakthrough, in at least the world of simulation?
Yes, I believe, quantum computing will eventually allow computations to be carried out that we can only dream of now, or we have not even thought of yet.
Well, Junko, it's been a great pleasure spending this time with you. Thank you so much.
Well, thank you again for taking the time.