Michael Peskin

David Zierler:

This is David Zierler, Oral Historian for the American Institute of Physics. It is April 27, 2021. I'm delighted to be here with Dr. Michael E. Peskin. Michael, it's great to see you. Thank you for joining me today.

Michael Peskin:

I'm glad to be here.

Zierler:

Michael, to start, would you please tell me your title and institutional affiliation?

Peskin:

I'm a professor at the SLAC National Accelerator Laboratory. My official title is Professor of Particle Physics and Astrophysics.

Zierler:

Now, what does professor mean at a national laboratory? And what might be the difference between professor at SLAC and at Stanford University's Professor of Physics?

Peskin:

There is a special relationship between the SLAC laboratory and Stanford University. When SLAC was founded, Sidney Drell and Wolfgang (Pief) Panofsky made a deal with Stanford that there would be faculty members at SLAC. Originally, there were supposed to be 20 faculty members. That number has grown to include accelerator physics and many disciplines of photon science. These faculty members are formally Stanford professors whose “school” in the university is SLAC. They are members of the Stanford Academic Council, which means that they directly can supervise students for Stanford Ph.D. theses. Their appointments are approved by the Stanford Advisory Board, which is the high-level board that approves all professorial appointments. That requirement sets a quality floor, that you have to be recognizable by this committee to be approved. Our ability to confer the title of Stanford Professor has allowed us to attract some very, very good people. Aside from the supervision of graduate students, there is no teaching obligation. But it is possible to teach courses by courtesy in the Physics Department and other departments.

Zierler:

This original arrangement, as I've heard, caused some tensions in the beginning between SLAC and the department. By the time you got to SLAC, had that legacy dissipated at all? Or were you feeling that in the early years at all?

Peskin:

This arrangement was not the cause of the tension. The cause of the tension was that the Stanford Physics Department felt that SLAC should be a part of the Physics Department, while Sid and Pief felt that SLAC was much too large and complex to be administered in that way. It had to be an independent entity. The original participants said, “This separation will happen over my dead body," and that's pretty much the way it worked out. Until the leaders of the Physics Department in the 1960’s had all left or passed away, there was still a barrier between the two institutions. I give a lot of credit to Douglas Osheroff and Steve Chu, who came into the Physics Department from outside in the 1980’s, in repairing this situation.

But when I came to Stanford, I was in a very lucky position. In the 70’s, the Stanford Physics Department made a big effort to recruit a new person in theoretical physics. They courted Steven Weinberg, who came to Stanford on sabbatical in 1976-77. But Steve did not accept the offer and got a nice position at Harvard. They then hired Lenny Susskind. I happened to know Lenny well, since he had been spending a lot of time at Cornell, collaborating with John Kogut, when I was a graduate student there. When I came to SLAC, we already had an established relationship. Lenny had also set up a study group on supersymmetry with Stuart Raby and Peter Nilles at SLAC. So we had an easy collaboration between SLAC and Stanford. Of course, SLAC was a center for elementary particle experiment, and so Dave Ritson and Stan Wojcicki were spending considerable time at SLAC. I had a very friendly relationship with Dave, and I adopted one of his graduate students, Leslie Rosenberg, for lessons on QCD. Still, with the elder members of the department, there was a lot of hostility, and that continued into the 80’s, until all of those people retired.

Zierler:

A much more contemporary question before we go back and develop your personal narrative. There are some wobbling muons right now that have a lot of people at Fermilab quite excited, and it's just been fun doing some insta-polling over the past week or so to get people's speculative takes. Based on what you know so far, does this look like new physics to you or not? [ About two weeks before the interview, the Fermilab muon g-2 experiment had reported a 4.2 sigma discrepancy with the prediction of the Standard Model.]

Peskin:

This takes us a little off the subject. There is also the complication that this is a history interview that people might look at 10 or 20 years from now. So you are asking me to bet on the final outcome. I'm going to put my bet that the muon g-2 measurement will be reconciled with the Standard Model.

Zierler:

So 4.2 sigma is not impressive to you?

Peskin:

You have to look at how the sausage was made. I think the experimental result could well shift by one sigma in the direction of the theory. Actually, it already shifted one sigma toward the theory in the recent announcement. There are new evaluations of the hadronic vacuum polarization from lattice QCD, which tend to move the theoretical prediction toward the experiment. So that 4.2 sigma could easily turn out to be 2 or 1.5, and then it will just be a statistical error. I’ll put this bet on the record. When people read this in ten years, they'll know whether I was right or not.

Zierler:

A very broad question, sort of a sociology of science question: Your career path is one that is representative more broadly of how people in particle theory went on into interests in astrophysics and cosmology. When did that roughly start for you, and what was that broader interest that brought you into these fields?

Peskin:

I wouldn't say that I'm working actively in particle physics and cosmology. It is a general trend now to move toward this area. The situation was different when I started as a graduate student. At that time, I wanted to cultivate very broad interests. Actually, when I came to Cornell as a graduate student, I didn't know whether I was going to do particle physics or condensed matter physics. And while I was there, I tried to attend both sets of seminars and keep abreast of what was happening in both fields. So at that time, I decide that trying to be an expert also in astronomy and astrophysics was too much. And that, it turned out, was a bad decision, because many interesting things were happening in astrophysics at that time. The things that made the biggest news at that time were the discovery of the very active objects—-pulsars, X-ray sources, active galactic nuclei. But this was also the time when it became clear that the matter content of the universe was dominated by dark matter, a new type of matter made out of new elementary particles. It took me a lot of time to catch up to the importance of this fact for particle physics.

The original evidence for dark matter goes back to the 30’s. But it really became clear in the late 70’s, from the observations of the Andromeda galaxy by Vera Rubin and Kent Ford, later confirmed with many other galaxies, that galaxies had associated matter that was invisible and very weakly interacting. Most of the matter in the universe, we now know, is in that form. Some particle physicists took up this issue quickly. Steven Weinberg and Ben Lee were some of the early participants, asking whether this matter could be heavy neutrinos. In the early 80’s, supersymmetry was proposed as the origin of dark matter. Actually, supersymmetric particles are still an important candidates for the identity of the dark matter, but many other possibilities are under consideration.

I didn't really follow this subject actively until the 2000’s. At that time, I got interested in a question which arose from my other interests: If you really understood the correct model of physics behind the Standard Model, for example supersymmetry, could you predict the amount of dark matter, and could you use that as an observational test that you had understood the whole picture? So I started to work on dark matter in the mid-2000’s, very much later, I think, than most other people in the field. Recently, there has been an explosion of interest in the possibility of light, very weakly coupled dark matter. This is something that I follow, but I'm not actively working on it.

Zierler:

So is this to say that with all of the excitement surrounding inflation in the early 1980’s, with people like Alan Guth demonstrating that a background in particle theory could be useful in cosmology, this also was not particularly compelling to you?

Peskin:

I should also have mentioned inflation. Inflation was invented at SLAC by Alan Guth, in 1980, a few years before I came there. I had a little involvement in the lead up to inflation. One of the problems that inflation solved was the “monopole problem”. If you have a phase transition in the early universe, at temperatures of the energy scale of grand unification, you will produce magnetic monopoles. And then, there would be many too many surviving to the present day, given the very strong constraints on the cosmic density of monopoles, so you had to get rid of them in some way. The problem was pointed out by Thomas Kibble and John Preskill. Preskill was, at that time, a graduate student at Harvard. I was a post-doc at Harvard then, and he was in the group of people that I was talking to every day. And so, I helped him refine the ideas that went into his paper. But after Guth solved this and other problems with the theory of inflation, I decided that was not going to try and develop it further myself. My research was going in another direction. And I don't know, maybe that wasn't a good decision. You have to choose to do one thing and not choose to do another.

Zierler:

Let's go back further in time. Let's go back to Philadelphia, and we'll start, first, with your parents. Tell me a little bit about them and where they're from.

Peskin:

I'm in the fourth generation after emigration from Lithuania. My great grandparents came over from Lithuania and Russia, the area that was called the Pale of Settlement. They were part of the major Jewish emigration from Russia around the turn of the 19th to 20th century. They were peasants in Lithuania, and they became farmers in New Jersey. My grandparents owned stores in West Philadelphia, which, at that time, was all Jewish. They did relatively well in the Depression because they owned shops. Both families strongly stressed education. My parents were both top scholars in high school in that area, and they both went to the University of Pennsylvania.

And then, they both became MDs. Actually, my father, Gerald W. Peskin, was a very well-known academic surgeon. He held appointments at the University of Pennsylvania, the University of Chicago, UC San Diego, and Yale. And at the very end of his career, he supervised the surgical residency program here in the Bay Area at Highland Hospital in Oakland. My mother became a general practitioner. Today we are concerned with other aspects of diversity, but, at that time, there was a lot of anti-Jewish prejudice. My mother's experience when she applied to the University of Pennsylvania Medical School was that she was told, “We cannot admit you. We already have one Jewish woman." She got her M.D. from Women's Medical College in Philadelphia. She got a great education there. She made her career half-and-half as a practitioner and as a homemaker.

Zierler:

But she maintained her practice, remained active as a doctor?

Peskin:

Well, as I say, halfway. So she would be, for example, the medic on call in an office building. And that would be a part-time position. After all of her kids were grown and my parents moved to San Diego, she took a psychiatric residency. For a while, she did emergency room work and, at the end of her career, worked for the Social Security Administration as the MD evaluating applications for disability.

Zierler:

And how long were you in Philadelphia?

Peskin:

Until I was about 16.

Zierler:

What neighborhood?

Peskin:

In the suburbs. Merion.

Zierler:

Was science, from your parents, something that was palpable in the household?

Peskin:

Yes, certainly. My father, being a surgeon, was working very long hours. This is unavoidable in that profession. As a professor of surgery, he also did research, inventing operations by operating on the stomachs of dogs. I spent time in his lab, but I never got deeply involved in it. I was interested in science at the level of reading about it, reading very widely. Everyone, at that time, had a chemistry set and did things in their basement that you wouldn't let your kids do today. But I wasn't taken up by laboratory science. I never really got the bug for it. Some people love to be in the lab, they buy glassware, they set up things, they love debugging experiments. I never got the feeling for that. So my career went in another direction.

Zierler:

Was your family Jewishly connected growing up? Were you members of a shul, Pesach Seder, that kind of thing?

Peskin:

Yes, but it was more a family thing than a religious thing.

Zierler:

Were you bar mitzvahed?

Peskin:

Yes, I went to Hebrew school and was bar mitzvahed. But my family was more the twice-a-year synagogue attenders. The Seders and other events were big family events. We had a big family in the Philadelphia area — three siblings on my mother’s side — and each family would host one major holiday. When we moved out of the Philadelphia area, we lost the family ties.

Zierler:

You stopped yourself, it almost sounded like you were about to say, "When we moved out of the shtetl." [laugh]

Peskin:

No, no, no, it wasn't a shtetl by then, [laugh].

Zierler:

What kind of schools did you go to growing up?

Peskin:

Public schools all the way. But these were top suburban public schools. I went to some famous high schools, Lower Merion High School, and then New Trier in the Chicago suburbs. Actually, there was a New Trier West at that time, and I was in one of the first classes there. Later, the New Trier system contracted back, and the site is now a community college.

Zierler:

And then, where did you finish out high school?

Peskin:

New Trier West.

Zierler:

You must've done very well to be accepted to Harvard as an undergraduate.

Peskin:

Yes.

Zierler:

Did you apply anywhere else? Or that was a simple early decision for you?

Peskin:

I don't remember, but I did apply there early decision and got accepted.

Zierler:

What was it like getting to Harvard in 1969? It must've been quite exciting with campus protests and everything else going on.

Peskin:

Well, that was a really strange time, actually. There was a large amount of political activity, but I also fell in with a group of freshman who were very serious about math and philosophy. The experience was very intense. My freshman year was the year that finals were postponed after the invasion of Cambodia. For freshmen at that time, everyone lived in Harvard Yard. And then, sophomore year, you would go to a house. I was there the year that they opened the Radcliffe houses to men. So I went to live in North House (now Pforzheimer House). That was a great experience for me because it turned out that this was the musicians' dorm. So that is where I learned about classical music.

Zierler:

Now, was physics the plan from the beginning?

Peskin:

No. The plan from the beginning was biochemistry. Because I thought that that was where science was going. I'd read a lot about biochemistry, and it seemed that that field was poised to make great progress. (This turned out to be true.)

At Harvard, they have special advanced courses for freshman—one in Physics, one in Math, and one in Chemistry. The chemistry sequence is a two-year sequence with physical and organic chemistry. That was a fabulous course. One of the best courses I've ever taken.

Zierler:

And organic chemistry did not scare you away from chemistry?

Peskin:

Oh, no, no, no. I wanted to learn that. And I had a great professor, a fellow named Weston Thatcher Borden, who idolized Robert Woodward. Do you know that name?

Zierler:

I do, yes.

Peskin:

He was the great synthetic organic chemist at Harvard for a long time. Borden would end each semester by leading us through one of Woodward's syntheses. It would go along with reactions that we had learned, and then there was a magic trick and the molecule would curl up in exactly with right way. The first term, physical chemistry, was taught by Dudley Herschbach. Both of these professors loved molecular orbital theory. And so, that's how I learned quantum mechanics. I didn't learn quantum mechanics through physics. And that turned out to be the right way, with less formalism than in physics and much more quantum intuition. That was a big advantage for me. (I continue to recommend the book that Herschbach used, a book by Charles Coulson titled “Valence”.) Given that I had this heavy chemistry course, I decided not to take the advanced physics course. I took the second-rank physics course instead. We did get to use Edward Purcell’s book on electricity and magnetism, still the best introduction to that subject. I did decide to take the advanced math course, Math 55. And that convinced me that I could not do super-abstract mathematics.

Zierler:

Who were some of the standout professors in the physics department for you?

Peskin:

Let me just finish the story. I studied hard on the route to becoming a biochemist, but I didn't succeed as a lab scientist. I never got the taste for working in the lab. And at that time, you couldn't be a biochemist unless you were a real chemist at a lab bench. So then, I tried to figure out how I could do more math and less lab, and eventually I migrated into theoretical physics. This was halfway through my time at Harvard. So I missed the standout professors for many of the courses. I never got the chance to take a course from Julian Schwinger. He left Harvard halfway through my time as an undergraduate, just at the time that I transferred to physics. I didn't meet Sidney Coleman until my senior year, when I took his famous quantum field theory course and audited his course in general relativity. It is well known that he was the super professor who explained everything with unmatched elegance.

When I transferred to the physics department, I decided, "Hey, I've had freshman physics, I’ve learned some quantum mechanics. Why don't I just dive into the deep end and take the graduate course in quantum mechanics?"

Zierler:

How did that work out?

Peskin:

I just barely survived, but it was a great experience.

Zierler:

Do you remember who taught that course?

Peskin:

Arthur Jaffe. Parts of it were very, very rigorous in terms of mathematics. I got some benefit from the abstract math course I had taken as a freshman, and that gave me a little advantage. Also, I had spent some time on my own working through some of the upper-class undergraduate textbooks. It worked out for me, then, just skipping the whole undergraduate program, trying to catch that up on my own, and going right into the graduate courses.

Zierler:

When you were thinking about graduate schools, you were firmly rooted in theoretical physics at that point?

Peskin:

Yes. By the end of the year after my transfer, all I was doing was theoretical physics. Not only the courses, but doing a lot of reading on the side, catching up on all of the classic Einstein papers that I should have read before. I was very serious about it. One of Schwinger’s remaining graduate students hung around my dorm, since his girlfriend lived there, and he encouraged me to read Schwinger’s paper on the Coulomb Green’s Function. It was an excellent experience, verifying all of the equations. It took me about a month. (The paper is 2 pages long.)

In the middle of my junior year, I met Alan Luther. Luther was at that time an assistant professor; later, he moved to the Niels Bohr Institute. At that time, I was interested in condensed matter and many-body physics. He introduced me to the theory of magnetism and suggested some exercises for me to work out. This was an important step for me. I also audited his graduate statistical mechanics class.

One day, he came into the class and apologized: ”I’ve been up all night studying this this new paper by Ken Wilson and Michael Fisher, and it has to do with the theory of phase transitions. I can't really explain it to you. You take a scalar field, and you add some nonlinearity with a phi^4 term.” I do not remember the rest, but obviously something very important had happened. This was my first glimpse of Wilson's theory of critical exponents, for which he later won the Nobel Prize. And so, I got really interested in that, and from that time, it was kind of my ambition to go to Cornell and study with him. And at that time, I was mainly oriented towards statistical mechanics, so I thought that's what I wanted to do.

Zierler:

That's a very well-defined goal as an undergraduate for what you want to do in graduate school.

Peskin:

Well, I'd been heading in that direction because I started in biochemistry. And then, I was looking at atmospheric physics and fluid dynamics. I had a great course in fluid dynamics. Again, I'd skipped the undergraduate course and went to the graduate course by George Carrier. And I learned so much. So there was fluid dynamics, and the problem of turbulence, and I got interested in that, and thought that maybe I could do something in that area. And maybe these new methods for statistical mechanics would help with that. So that was the kind of thing I was thinking about when I applied to graduate school.

Zierler:

Now, were you in contact with Wilson before you got to Ithaca? Did you have a commitment from him that he would be your advisor?

Peskin:

Well, no. But I think I mentioned this in my application, and when I visited Cornell, he made a point of coming to talk to me. And he's a very interesting character.

Zierler:

What were your impressions of Wilson when you first met him?

Peskin:

Well, he's kind of a little strange. He talks in a halting way, he's very thoughtful. A little detached from reality. Incredibly brilliant. I didn't even begin to appreciate it then. But it was like, "Here's the guru, and he's going to take you somewhere." And that was very interesting to me.

Zierler:

What was he working on at that moment when you first connected with him?

Peskin:

I'm not quite sure. It was the period when the phase transition problem had gone on to another group of people. And he was more interested in other questions. I think he was working on the Kondo problem at that time. And he was on the way to lattice gauge theory, but he hadn’t yet gotten there. He supervised a thesis by a student of his named Ashok Suri on the use of phase transition concepts to define quantum field theory, which was part of Wilson's big endeavor. So he must've been thinking about field theory on a lattice at that time. I don't really remember what we talked about then. I didn't meet him again until I actually showed up at Cornell in September of 1973. Between the winter and the fall of 1973, a lot of things happened. There was another set of things that I learned about when I was kind of wandering around the halls of the Harvard Physics Department. In my junior year, my TA in quantum mechanics, Eric Weinberg, pulled me aside and said, "There's this amazing thing that's happened, and you've got to learn about it. Probably, it'll be over your head, but you've got to go to this lecture that Álvaro de Rújula's going to give." De Rujula explained the revolution in gauge theories and spontaneously broken gauge theories. It took me some time to figure out what that was about, but it was obviously something extremely important.

And what happened in 1973, in the spring of 1973, was the discovery of asymptotic freedom. There was a big ferment, of course, at Princeton and Harvard at that time. David Politzer was a graduate student then, and I was also a TA, so he was one of the people that I would meet from time to time. Actually, I TA-ed in the same course with him that summer (1973). And he was complaining, "I've discovered the theory of strong interactions, and they won't even give me a decent salary." That's just when he got his PhD. But then, he became a Junior Fellow, and the rest is history. With all of this, I'd learned about gauge theories in my senior year. And then, when I came to Cornell, very soon after I got there, Ken Wilson gave the first seminar at Cornell on lattice gauge theory. And his attitude was, "Enough of this statistical mechanics stuff. It's time to solve the strong interactions." And so, that's how I became a particle physicist.

Zierler:

Now, did you miss all the excitement around grand unification at Harvard during your last years there?

Peskin:

I didn't really know enough to participate in that. In 1972, there was the famous Georgi-Glashow paper. I wasn't connected enough to know what was going on there. Obviously, there was a lot of ferment. I was not able to get involved in it.

Zierler:

What was the intellectual process for you as you were thinking about your own thesis research?

Peskin:

Well, that, as I say, evolved when I came to Cornell. I had a big decision to make, whether to go into particle physics or into condensed matter physics. And both of them were very appealing. Michael Fisher was the big leader in phase transition theory at that time at Cornell. And he had a very interesting program. David Nelson was his graduate student at that time. And David was obviously going great guns in that direction. But I decided not to follow them. I thought I'd place my bet that wherever Ken was going was going to be interesting.

Zierler:

And what was Wilson's style as an advisor? Did you work closely with him? Did he give you a problem and send you off on your own? What was that dynamic like?

Peskin:

Well, that was a little weird. He did give me a problem, and I’d meet with him every week. And when I was working intensely on that problems, it was like, it would consume me, certainly the few days before I had to see him, and then I'd go see him, and we'd talk about the calculation. He'd ask me questions. It took me some time after the end of the interview to figure out what he was asking me. And it always turned out that he wasn't answering my questions, he was giving me the answers to the questions that I should have been asking, given my results. It would take me a few days to figure that out. And then, I'd keep going.

Zierler:

Now, is your sense that this is because interpersonally, he was awkward? Or was he just operating on a different level?

Peskin:

Both. But a lot of the latter. I got to understand this better when I became a thesis advisor. Very often, you give a graduate student a problem, and you wait and see what comes back. And you predict what's going to come back the next week. You say, "Oh yeah, that's what I thought you'd get. Yeah, the curve is really supposed to look like this. Why don't you just check and see that there's an inflection point here?" And then, they would think, "Oh, gee, you're a professor, you're so smart. You knew that that was there." Finally, they come with something that you weren't expecting, and that's when the research really begins.

Zierler:

Was there anything going on in experimentation that was relevant to your thesis research?

Peskin:

Well, no, except that Cornell, at that time, was an active laboratory in particle physics. I had an office from the day I got there, not in the main physics building, but in the Newman Laboratory of High Energy Physics, which is a somewhat older building behind the main Physics building. The physicists have all moved out by now, and I’m not sure what it's being used for currently. But at that time, all of the particle physics activity was in this building that had been there since the 50s. When Robert Wilson was there in the 50s, they formed the group. There's an electron accelerator that dates from the 60’s at Cornell. There was an active experimental program there. And, as a graduate student with an office in that building, I'd hang out with all of the experimentalists. And then, you just couldn't help but learn a lot about experimental physics. In the situation of being a theorist, I didn't actually have to get the electronics to work, but I knew things those guys didn't know. So that was a good source of interaction. And I think I got a very good education in how particle physics is done that has served me very well.

Zierler:

Besides Wilson, was there anyone else on the Cornell faculty who served as a mentor to you?

Peskin:

There were many top people there, and I tried to learn from everyone. Kurt Gottfried, Toichiro Kinoshita, Tung-Mow Yan. John Kogut was there as an assistant professor, and this was the time when he was very active. And so, I had a big interaction with him. Unfortunately, there was also a competitive aspect to it. He didn't eventually get tenured at Cornell. I have a feeling he felt a little threatened by me at the same time that we were talking a lot about physics. Lenny Susskind was collaborating with John, so he spent a lot of time there. He was a big influence. I think most of what I know about gauge theories, I learned from Lenny.

Zierler:

And Lenny was on the faculty at that point at Cornell?

Peskin:

No, no, Lenny went through in a series of positions in other places. He was at Yeshiva University, and he had an appointment in Tel Aviv. So Lenny was still working his way up. For a long time, people didn't understand how smart he was or how important his ideas were. And so, he never had a position at what you'd think of as a top-ranked university. But he had a big influence. Gradually, he worked his way up the academic ladder. Finally, in 1977, he got the professorship at Stanford.

Zierler:

What were some of the issues that you talked about with Lenny?

Peskin:

Lattice gauge theory, quark confinement, how you understand quark confinement. Somehow, we didn't talk a lot about supersymmetry. That was another thing that was in the air in the 70’s that I didn't go into strongly at that time.

Zierler:

Was he, in those early years, following Veneziano, and Schwarz, and Green? Or he came later to that?

Peskin:

Gabriele Veneziano wrote the first paper on string theory. Veneziano developed the theory as a theory of the S-matrix. Lenny, in 1970, was one of the people who realized that Veneziano’s theory described a relativistic string. You know this story, right? There was a problem in S-matrix theory that people were trying to solve. Richard Brower, who was one of the other early participants in this, told me, “We were all going through the Bateman Manuscript Project.” Do you know what that is?

Zierler:

No.

Peskin:

Bateman was a math professor at Caltech. He was an expert on special functions, and he left behind boxes special functions integrals and identities. Then, his students and colleagues organized this into a multi-volume series of works on special functions called the Bateman Manuscript Project. Geoffrey Chew and the supporters of S-matrix theory had come upon a special property of hadronic scattering amplitudes called “duality”, in which the S-matrix amplitudes could be written as a sum of s-channel resonances or, alternatively, as a sum of t-channel resonances. (In quantum field theory, a sum over both types of resonances would need to be included.) Then the question could be posed, can one write an analytic expression for an S-matrix element with duality. As Richard Brower told me "We were all going through the Bateman Manuscript Project page by page to try to find the right function, and Veneziano was the first one who got to the beta function." Veneziano is an amazing person, and, yes, he was the first to find the expression that had all of the right properties. After this, others tried to generalize the expression. Ziro Koba and Holger Nielsen found a generalization to scattering of any number of particles. So the scattering amplitude was known, but it was still a mystery what was the physical picture behind it.

Then a number of people, including Lenny and Yoichiro Nambu, realized that you could derive this structure from something like a harmonic oscillator. Looking into the details, it was exactly the expression for a relativistic strong. I don't know how important that was, the development of the theory, because a lot of things, you could just do from the amplitudes and the harmonic oscillator picture without the physical picture of a string. I think that the inclusion of fermions, by Andre Neveu, John Schwarz and Pierre Ramond, predates the string picture. But of course, it is a complete change when you know you're doing physics rather than just playing with integrals. That was in 1970. After this, Lenny started working on the infinite momentum frame, the simplifications in field theory when you have highly boosted particles. He realized that this was connected to the parton model and the ideas people had about hadronic structure at that time.

Zierler:

For your own research, how did you know you had enough to defend? Or did Wilson make that decision for you?

Peskin:

So I didn't finish explaining this, but my relationship with Wilson was kind of off and on because I was interested in a lot of stuff. And it was very distracting from actually getting anything done. Ken has a strange history that he never published any papers. He didn't publish his thesis. He spent a year at CERN, and at that time, he published two papers. And those were the almost only papers he published until the late 60’s. There were papers in '64, but it's still a very meager publication record. From this, I took the wrong advice that it was fine not to publish papers and to very perfectionistic about what you did publish. I am lucky that I survived that.

Ken did spend a lot of time refining his ideas. Ken had a big paper introducing what we now call the Operator Product Expansion that he submitted to the Physical Review in 1964. It came back with a long referee's report, which was important enough that he preserved it in his papers. It's an interesting read, actually. The referee, it's now known, was Arthur Wightman. There is a lot in the report about how the paper pretends to be mathematically rigorous, but really it isn't, so here are some things that you should do, but it also had some substantive suggestions. The most important ones was to look at the Thirring model, which is an exactly solvable two-dimensional model that Wightman thought Wilson could use as an illustration of some of the points in the paper. But actually, the Thirring model was also a counterexample to points in the paper. So this led Wilson on another several-year quest, and this paper was never published. The Thirring model has anomalous dimensions, which is a very important concept. I don't know for sure, but I think Ken woke up to the idea of anomalous dimensions by studying the Thirring model. It is well documented that he spent a lot of time trying to understand the lessons of this model. It was 1969 before he was ready to publish. There were two papers, but one of them never got published. The other one is a paper with a very odd title, “Non-Lagrangian Models of Current Algebra”. That paper gave a large number of practical applications of the ideas of the Operator Product Expansion and anomalous dimensions to particle physics. And this just made a revolution in quantum field theory.

So that's how Ken got started. But a very important part of, as it were, the persona was the idea that you didn't publish papers unless they were really important. And unfortunately, I got the bug, and I've never lost it, even though what you really should do as a theoretical physicist is just the opposite. Publish all of your ideas, and see which ones float and which don't. So I was working on this problem, and I just could never bring it to a conclusion. And I didn't get the pressure that I should have from Ken, "OK, already, you've got to finish this work, write it up, and go onto something else." And so I went on meandering for several years. I didn't have anything published, actually, even at the time of my PhD. But it was time for me to go someplace else. I'd learned what people at Cornell had to teach me, and I needed to go someplace else. And Ken apparently wrote me an off-scale recommendation letter for the Harvard Society of Fellows. I botched the interview, so that's the only way I could've gotten in there.

Zierler:

Besides Wilson, who else was on your thesis committee?

Peskin:

I think it was Kurt Gottfried and Karl Berkelman. Karl Berkelman was a senior experimenter. He was eventually the director of the Cornell synchrotron lab. Very hard-nosed experimentalist. He never really thought I was serious, I think. And that was actually correct; I couldn't live up to his standards as an experimenter. But he was a good influence because his standards were very high.

Zierler:

When you got back to Harvard, did it feel very different from undergraduate? The Society of Fellows was a totally different world? Or not so much?

Peskin:

The department was totally different. Actually, along the way, I'd met some of the characters. A big experience for me was going to the Erice summer school in 1975. The lecturers include Sidney Coleman, Ken Wilson, Gerard 't Hooft, David Gross, Sid Drell, and Carlo Rubbia. If you look at those proceedings, it's the very top people in the field lecturing at that school. So I met all these people. When you're nobody, and then you've met these people, now you're one rank higher. And certainly, the Sidney that I met there was very different from the one I knew by sitting in the back of the classroom and listening to his lectures. It was a different kind of relationship. And, I did have a certain status being a Junior Fellow. And actually, also, it turned out that I knew things they didn't know.

So this was good. One of the things that I'd done as a graduate student was to make a big study of chiral symmetry and from a mechanical point of view. The Harvard professors were all interested in chiral symmetry from the symmetry point of view. But I had gotten interested in something called the Nambu-Jona-Lasinio model, which was very much out of fashion then. Yoichiro Nambu and Giovanni Jona-Lasinio, back in 1961, had a mechanical approach to chiral symmetry with strong interactions. Their paper was very under-appreciated for a long time. But it is actually a path-breaking paper. I think now, the viewpoint has changed, people understand it. This kind of mechanical way of looking at quantum field theory was something that was at the heart of Ken Wilson's program So it fit with other things that I was thinking about.

When I came to Harvard, people were interested in the axion, which had just been postulated by Steven Weinberg and Frank Wilczek, and they were trying to work out its properties. It turned out that I knew a lot of things relevant to this problem that most people there didn't know. So that raised my status. Again, there was a problem of publication. If you look at Steve Weinberg's paper on the axion, there are a lot of references to me, unpublished. Eventually, I wrote a paper missing one section, I think, that we were going to publish together, a Physical Review-length article on the axion. But by that time, Steve had convinced himself that the axion didn't exist. And so, this paper was never published. All of these things are very odd, but that's just the way my life worked.

Zierler:

Was your sense that Society of Fellows was a finishing school for would-be Harvard professors?

Peskin:

I don't know. I had a strange relationship to the Society of Fellows. Nowadays, the Junior Fellows actually meet, by themselves, for lunch once a week, and they get to know each other. And it wasn't like that then. The Junior and the Senior Fellows would just meet for the Monday dinners, which were rather formal. The dinners were fun. The Senior Fellows knew a lot and had all kinds of great stories. But it wasn't the center of my intellectual development. That was more in the physics department itself. There, I had a lot of great colleagues then. My officemate was Jim Gates. Ed Witten was two doors down the hall. Paul Steinhardt was graduate student, then he became a Junior Fellow. As I said before, John Preskill was a graduate student at that time, and I also spend a great deal of time with Orlando Alvarez, who's done a lot of really beautiful mathematical physics. The three of us worked together very hard on the instanton, which was a new concept then. And again, that didn't yield a publication because I was the leader, so that wasn't good news for those guys. There were some pieces of the story that we couldn't figure out, and eventually it never got published.

Zierler:

What as Jim Gates working on when you were officemates?

Peskin:

He was working on superfield supergravity. Another important player there was Warren Siegel. You probably know him as a professor in Stony Brook. At that time, he had zero status. He had been a student in Berkeley working on string theory. No one was very interested in him. He decided to take the gamble to come to Harvard as a visitor with zero support. They gave him an office up on the fourth floor, out of sight. The first thing he did was to write a paper about the Wess-Zumino superfield formulation of supergravity, claiming that all of the physical degrees of freedom could be gauged away. That was an error, and then, for a long time, Bruno Zumino would never look at his papers. After that, he got together with Jim because Jim had worked on supersymmetry when he was a graduate student. And they worked everything out extremely carefully. Finally, they had their own formulation of supergravity in the superfield formalism, and it was especially compact and elegant. But I was off in another direction. I wanted to solve quark confinement. And so, I was working on very different things. But I learned a lot from those guys.

Zierler:

Who were you working on quark confinement with? Or was it mostly solo?

Peskin:

It was mostly solo. I was talking a lot to Paul Steinhardt. Of course, you couldn't help but talk to Ed Witten. He was coming up with a new idea every week. But yes, I was really struggling with this. What's the formalism for confined quarks? How do you calculate their spectrum? There still aren't good answers to those questions. I had some good ideas, but they were never fully developed. But there wasn't progress in that direction. Eventually, I had some breakthroughs. I went to Cornell for the summer. My girlfriend was still doing her PhD at Cornell. (We got married about a year after I came to Harvard.) And so, I was visiting Cornell all the time.

That summer, I got involved in a problem that I think Kurt Gottfried proposed about the QCD potential for the quarkonium system. And that led to some interesting papers about what is now called the multiple expansion in QCD. After my papers, Tung-Mow Yan and Yu-Ping Kuang developed this and kind of made it a predictive formalism for some of the B-spectroscopy experiments people were doing then. And then, at some point, I got interested in technicolor, and that got me started in physics beyond the Standard Model. In some sense, I've been doing research on Beyond-the-Standard-Model continually ever since that time. The questions that we had at that time are still not answered. Models keep being proposed and being ruled out. And we don't know what the answers are. I got started on that when I was at Harvard.

Zierler:

Michael, tell me about your time at Saclay.

Peskin:

Well, that was a great experience. I forget how I got connected to Édouard Brézin. I think it was probably through statistical mechanics. He was visiting Cornell, I got to know him then. I had to go to Europe because my wife was a German literature scholar, so she wanted to take a year at some place in Germany. She decided to go to the University of Münster. So we took out a map of Europe and figured out what would be the closest place that would be really good for physics. It was either Hamburg or Paris, both of which are quite some distance from Münster. But I'd met some people from Saclay, and I knew it was a great place, and I decided to make that the target. For the Society of Fellows, you could take one year to visit some other place, and they would pay your salary when you were there.

Zierler:

Did you see this as an opportunity for new collaborations, new topics to be exposed to?

Peskin:

Certainly, new topics. But at that time, I was still going solo. But if you go to a new place, you learn what's going on there, and you get new ideas. I did actually collaborate with the Saclay people. We wrote a great paper called “The Roughening of Wilson's Surface”, about the roughening transition in lattice gauge theory. So that actually had some influence. Actually, when I came back, I gave a lecture at Columbia about this paper. And I met, maybe for the only time in my life, I. I. Rabi, who came into the coffee room before the seminar specifically to ask me the question, "Hey, Peskin, how do you polish a Wilson's surface?" It's a very Rabi-type remark. But mainly, at Saclay and afterward, I was working on my own stuff on technicolor.

I gave some lectures at Saclay on beyond the Standard Model. That was a big wake-up call for them because they'd mainly been working on statistical mechanics and QCD. I wrote, basically, the third thesis on lattice gauge theory. The first one was written by my colleague Belal Baaquie, a fellow Cornell graduate student. The second one was written by Jean-Michel Drouffe, who was a graduate student of Claude Itzykson at Saclay. So there was a connection there. I'd met Itzykson before I went to Saclay. So we had a lot of discussion of those things. The Saclay group was not thinking much into beyond the Standard Model. In fact, beyond the Standard Model was a whole new field then. People were still accepting that the Standard Model was established. So why would you even think about going beyond it?

Zierler:

From your perspective, what were the theoretical underpinnings at this very early stage that the Standard Model should be broken, should be explored beyond?

Peskin:

It's the same question we have today. The weak interactions are based on spontaneous symmetry breaking. If you know anything about statistical mechanics, you would say that there has to be a physical picture explaining why the symmetry breaks. You don't just write down a potential from nowhere and say it has the right shape, and that's the end of it. There should be a physics explanation. And where can that come from? Technicolor, invented by Lenny Susskind and Steve Weinberg, was a big step in that direction because it was a physical model that actually was able to predict that SU(2)xU(1) should be spontaneously broken. The ingredients that caused the symmetry-breaking are not contained in the Standard Model , so it had to involve a new physical interaction.

So there's some new force of physics that's out there that we haven't discovered yet, but which is essential to making the weak interactions do what they do. And isn't that just the most important question in science? Maybe not in science. Maybe consciousness is the most important question. But in physics, this is definitely the most important question. A lot of people at that time in the late 70’s, when it was becoming clear that the Standard Model was right and was standard, felt this was a very important direction. For me, because of the Ken Wilson influence, I was convinced that quantum field theory is just quantum mechanics, so everything has a mechanical explanation, and you just have to find it. From that perspective, this idea is totally compelling. And, in some sense, I've never lost that. I'm still working on the question of why the weak interaction symmetry should be broken. And we still don't have the answers. So we've got to keep working on that, probably for a long time.

Zierler:

You mentioned how much Harvard had changed. And so, I can parallel that question when you got back to Cornell as a visiting assistant professor. Had Cornell changed since you were gone as well?

Peskin:

Well, not so much because I had that same relationship with the people there. As I said, I was continually going back and forth between Boston and Ithaca in the years that I was away. And so, I kept in contact with those people. I suppose that, as I developed as a physicist, they respected me more. I wasn't always this guy who just shoots his mouth off and sometimes knows what he's talking about, but sometimes doesn't.

Zierler:

Now, was the title Visiting Assistant Professor to indicate that this was more than a post-doc, but they did not have a tenure line open for you at that point?

Peskin:

Exactly. In fact, I told them not to open a tenure line. Because the plan was, as soon as my wife got her PhD, we'd move somewhere else.

Zierler:

Ithaca was not a long term goal for you.

Peskin:

Had my life been different, it would've been great. As I said, there was this big issue of John Kogut's tenure, which I did not want to interfere with.

Zierler:

But it didn't happen anyway.

Peskin:

It didn't happen anyway, but they got Peter Lepage, and that worked out fantastically for them.

Zierler:

When did you first meet Peter?

Peskin:

I don't know, actually. It may not have been until I came there in this Visiting Assistant Professor position. I knew about him because I'd met Stan Brodsky at some point. Peter was Stan's graduate student. And they were working on the theory of exclusive hadronic processes with large transverse momenta.

Zierler:

But you're saying you met Stan before SLAC?

Peskin:

Yes. You live in this community, you go to meetings, you meet people. And if you've met Stan, you don't forget him. Back when I was an undergraduate, when I transferred to the physics department, I was assigned as an advisor this new assistant professor, Tom Appelquist. I did get to interact quite a bit with him. He was a post-doc with Stan. In fact, what everyone was saying about Tom at that time was that he's the guy who solved the problem of the Lamb shift. This was a major calculation done in collaboration with Stan—-a landmark calculation. It was one of the first calculations that used computer algebra processing to do a very involved Feynman diagram calculation, to break the problem down to the point where you could evaluate it numerically. In that process, they discovered that the previous two-loop calculation of the Lamb shift had errors. And when you corrected the errors, the result from quantum electrodynamics was in excellent agreement with experiment. So that's how Tom originally made his name. So I learned about Stan through him.

And then, I got to meet Stan, and he's an amazing character. One of the things I did right before I went to Europe was to write a little paper that explained some questions that Brodsky and Lepage had about the results of their analysis. So again, it's interesting. If you haven't had a big discovery as a theoretical physicist, you can still get known because you know things that people don't know. And you can put them together with the other pieces of information, and it turns into something bigger. Actually, a lot of the work I've done is like that. Supplying some missing piece that then allows you to go forward from what other people have done. That happened in this case. So I met Stan through that. And, reading his papers, it was clear that Peter was going to be a major force.

Zierler:

Did you have any opportunity to do any teaching at Cornell?

Peskin:

Yes indeed. That's one of the reasons that I wanted the professor position, because I wanted to teach. I taught quantum field theory there. The notes for that course were the first version of my book. Also, I gave a course called “Topics in the Theory of the Weak Interactions”, which was mainly about beyond the Standard Model.

Zierler:

When did you feel like you were ready with your wife to start looking for longer term positions?

Peskin:

First of all, I made a big splash in 1981 at the Lepton Photon Conference. They were looking for a speaker on composite models of quarks and leptons. This was a very odd subject. There's really no good model. But people thought it was in the air, so someone should summarize it. I was asked to do this. And it actually went very well, the lecture was quite interesting, and, I think, very thought-provoking, although no problems were solved. I had a great experience at that conference. In the APS photo gallery, there is a picture of me and Lev Okun on the boat trip on the Rhine River at that conference. Lev was finally allowed to leave the Soviet Union after many, many years, and he went to CERN, and then he gave a talk at this Lepton Photon meeting. At that time, he was obsessed with a model of Haim Harari called the Rishon model, which was a constituent model of quarks and leptons. Lev was convinced that the Rishon model was garbage. And he wanted me to say this in my lecture.

So we spent more or less the whole conference arguing about this. What could you really say? What were the properties that this model was supposed to have? How do you think about the constituents of massless and almost massless particles? That's a whole subject in itself. So in particular, on the excursion, the barge trip, everyone was out watching the Rhine River, and Lev and I were sitting at this table, and he would say, "Oh, I told you this. Let me just try and explain it to you again," going over and over his ideas about the sub-constituents of matter. Eventually, I suppose, my wife dragged us out and said, "We're passing the Lorelei. You really shouldn't miss this." But in any case, that talk went well, and my name was circulated in the community after that. And then, I was getting a lot of offers.

Zierler:

Obviously, SLAC among them.

Peskin:

And my wife was deciding that maybe she didn't really want to be a German professor after all. So we set our time to leave Ithaca in the summer of 1982. and then, I spent that year doing a lot of interviews. Not a very productive year for me for physics. But that's the way it goes.

Zierler:

Who was most encouraging of you coming out to SLAC?

Peskin:

I'd met Sid Drell at the 1975 Erice school. And I think he had his eye on me from that time.

Zierler:

And the theory group at SLAC obviously was quite strong at that point.

Peskin:

Well, actually, no. The big influence in the theory group up to that point was Bj — James Bjorken. But in 1979, Bjorken left and moved to Fermilab. They needed someone to fill this vacuum, and it wasn’t clear who. Helen Quinn had just moved to SLAC from Harvard and was a staff member in the Theory Group, but she was more or less passed over. Instead, they decided to go with this bright young guy with big promise but not too many achievements. Anyway, it was very lucky for me that they offered me the position.

Zierler:

What were you working on when you first got to SLAC?

Peskin:

Technicolor. I was still trying to make technicolor models. The big question was, could you use technicolor to explain the quark and lepton mass spectrum? And I must say, I worked on that problem very hard. It was a total disaster. There's one little paper that I wrote that is somewhat interesting. Not so much came out of that.

Zierler:

What was so difficult about it?

Peskin:

You just don't have enough structure. You can't really solve the new et of strong interactions, so you can only work with chiral symmetries. They have to be spontaneously broken. But you don't know what the breaking pattern is, and you can't compute that from first principles. The one strong interaction theory that we understand well, QCD, has a very simple symmetry-breaking pattern. I guessed that this pattern would be general for “vectorlike” gauge theories, and now there is a lot of evidence to back that up — including a theorem by Edward Witten and Cumrun Vafa. But you need much more symmetry breaking to explain the structure in the quark and lepton spectrum. Even in chiral gauge theories, the patterns do not seem to be complex enough. Even trying to get hierarchies among the fermion masses is difficult within this context. It's just hard to make progress, and, yes, I didn't make much.

Zierler:

By the time you got to SLAC, this is the very beginning of discussions that would ultimately come to fruition with the planning for the SSC. Were you following those developments at all? Were you excited about what might happen if the SSC was built?

Peskin:

Yes, indeed. But let’s not go there yet. There were other things I was interested in at that time. I was interested in string theory. This is before string theory became popular.

Zierler:

Before the revolution in '84, you mean.

Peskin:

I'd learned about string theory when I was a student, and I was always interested in it because I'm interested in the mechanism of quark confinement. I thought that string theory could be a model of quark confinement and give us insight. While I was at Cornell, I gave a student there, Emil Martinec, a thesis problem on string theory. Orlando Alvarez was a postdoc at Cornell then, and he was working on string theory and confinement, so this was a natural direction. I gave Emil the problem of understanding the Polyakov theory of the supersymmetric string. That was in 1981, so it turned out to be very prescient. Emil’s work got him a postdoc position at Princeton, and he arrived there in 1984, just as the string theory revolution happened. He was ready to jump in. So that's probably one of my biggest successes as a thesis advisor. I was also thinking about heavy quark theory. But the big thing I was working on was technicolor, and it didn't go anywhere.

Another thing that I dipped into there was the theory of supersymmetric nonlinear sigma models. Under the influence of Jim Gates and Warren Siegel, I'd had to learn something about supersymmetry. And then, supersymmetry became big around that time, around 1980. I was interested in non-perturbative supersymmetry and did a little work on that. This subject got a boost when string theory became popular.

But now, let’s talk about the SSC and high energy colliders. One thing I didn't tell you about my graduate student experience was quarkonium. I didn’t actually write any papers on quarkonium when I was a graduate student, but it was a major discussion topic at Cornell. Ken Lane and Estia Eichten were post-docs at Cornell at that time, and they were working with Kurt Gottfried and Tung-Mow Yan on the theory of the charmonium spectrum. John Kogut was also working on that. So I learned a lot about it. One of the things that really impressed me at that time was how much you could learn about these systems from electron positron annihilation, relative to what you could learn from proton collisions.

This gave me an unusual perspective. Most of the experimental community in the US is centered on Brookhaven and Fermilab. For them, everything was about what you could do with proton fixed-target experiments, or, after the discovery of the Z and W bosons, with hadron colliders. The supercollider, the SSC, was an embodiment of that attitude. For this community, the obvious thing to do in the future is to build the biggest proton collider you could, and go to the highest energy you could. But I came out of this different culture. At that time, Cornell and SLAC were the two leading US electron laboratories. There, people thought, “We can work with electrons, while all of those other people are working with protons. They're just smashing things together.” A popular analogy at the time was to say that proton experiments are “like throwing watches together and seeing what comes out.” With electrons, you can do something different and, arguably, more powerful.

So even before I came to SLAC, I was interested in the question of what the future holds for e+e- colliders. I was at Snowmass 1982, which was when the community really started focusing on the supercollider energy regime. As I told you, I got very early onto the track of beyond the Standard Model physics, and I was convinced that this was where the field was going. Here again, I knew a lot of stuff that other people didn't know that was very relevant to the discussions taking place. However, because I was interested in electron machines, I didn't work very much on the proton colliders. On the other hand, there was large community interest in that subject, and you had to follow it. As the Tevatron experiments were prepared and the SSC was being discussed, proton collider physics became the most important topic.

A member of a national laboratory, you had to get involved in that. But oddly, in '84, '85, when the SSC was really anointed as the future of US high energy physics, the SLAC faculty decided not to join one of the experimental collaborations. They were going to go their own way. And I don't know, it's an interesting decision. What would've happened if we'd done the opposite? But eventually, of course, the supercollider failed. And I think, not for any reason of one laboratory joining or not joining. Somehow, it was just too big, and it wasn't sold correctly for its size.

Zierler:

When did you start paying attention to what was happening at CERN?

Peskin:

Well, having been involved in the discussions about the SSC, you couldn't ignore what was going on with the LHC. And as a theorist, you just take your computer program, you type in some different energies, and study the same things. So it was easy to follow that. Between the SSC and the LHC, there was a great deal of politics that probably you could spend your time better in this interview not talking about. But I think in '88, Rubbia, who was the director designate of CERN at that time said that they could build the LHC for a billion dollars and turn it on in 1997. And so, it was far ahead of where the SSC would be. The debate continued in the community. I remember a discussion I had Peter Jenni in 1993, just before the cancellation of the SSC, where I said, "You should take the ATLAS detector, put a big CERN stamp on the side, and build it in Texas. And then, we could unify the community." So I was involved in those discussions and trying to understand what was going to happen with these machines. It wasn't my big love because I was interested in this electron direction.

Zierler:

Tell me about when you started doing precision studies of the W and Z bosons.

Peskin:

I started to work on this because we were going to do these experiments at SLAC. The SLAC program on the Z boson, the SLC, began in 1989. A big influence for me was Bryan Lynn, who came first as a post-doc, and then became a Stanford assistant professor in the late 80’s. I was talking to him a lot. We wrote a joint collaborative paper, also with Bryan’s student Robin Stewart. It was actually published one of the CERN yellow books rather than in a journal. But this paper was very important in pointing out that you should study the electroweak radiative corrections as an effective theory, and that you could organize your thinking about them systematically. Bryan was educated to do the kind of full, hard calculations of radiative corrections. He's a student of Alberto Sirlin. I was never at a professional level in that subject, but I was interested in the more conceptual aspects. So that is how we collaborated. He knew how to do the calculations, but I had a better understanding of what they were good for and what they could test. But then, obviously, you had to really get in, and get your hands dirty, and study a lot to understand what the capabilities of the machines were.

Actually, I had been interested in Z physics back when I was at Cornell. Henry Tye and I organized a meeting on precision Z physics and the possibility of building a Z factory at Cornell. But when the SLC experiments were about to start, someone was needed to come in and explain the theory to the world in a way that experimenters could really use it. And I felt that was a job that I should do.

Zierler:

Your interests in the top quark were concurrent? Or this comes later?

Peskin:

That started with another project that I was involved in in the late 80’s. Burt Richter was interested in making a serious proposal for a TeV energy e+e- collider. In 1986, he set up a committee at SLAC to write a report on the physics that could be done with such a machine. David Burke and I were the conveners of that group. And for this report, we did the first simulations of what e+e- physics would look like at 500 GeV and 1 TeV. The results were very interesting. You can guess what the results will be, but actually doing the simulations made it very concrete. Dave, Tim Barklow, and Mike Hildreth, who was a student then and is now a professor at Notre Dame, wrote the first paper on the precision measurement of the Higgs boson properties at an e+e- collider. We put out a report in 1988, which is, I think, the first major report on the physics prospects for a high energy e+e- machine.

One of the questions that was asked through the 80’s was the question of what the mass of the top quark would be. From studies of the weak interactions of b quarks done at Cornell in the early 80’s, we knew that the top quark must exist. There were general upper limits on the mass of heavy quark at about 400 GeV. But where was it exactly? When the experiments at the CERN p-pbar collider failed to discover the top quark, we know it had to be at higher energy. Then the study of the top quark would be a part of the physics program of a future e+e- collider. With Matt Strassler, who was my graduate student at that time, we wrote an influential paper on how the top quark would appear at a high energy e+e- collider.

I had a lot of fun thinking about the physics of high energy e+e-. It is much easier to measure the important properties of elementary particles in this environment compared to the environment of pp colliders. So you could figure out how measure very difficult quantities, such as particle mixings and masses of particles that decay to lighter, invisible particles. Afterward, you could think about going to a pp collider, where you can't measure so many variables, and try and invent tricks that allow you to do almost as much. That's the way that the subject would naturally proceed.

Zierler:

Now, just to zoom out a little bit, in the 1980s, early 1990s, discussions relating to the search for the Higgs, did you see this as a capstone to the Standard Model or as the first step to physics beyond the Standard Model?

Peskin:

Well, I never believed the Standard Model was complete. So this idea that the Higgs is a capstone to the Standard Model was just totally outside my way of thinking. And maybe I should say, it still is, even though that's probably the way most people think today.

Zierler:

It's certainly a standard idea, if it's not correct. It's the prevailing way of understanding the Higgs.

Peskin:

Right. But I have already explained to you a couple times earlier in this interview that I think that's the wrong way of looking at it.

Zierler:

Yeah, let's flesh that out a little bit, the theoretical basis for this. Why is this your position?

Peskin:

Well, it's just what I told you before. The idea that you write down the Higgs potential by fiat, and then you claim to have understood why electroweak symmetry is broken, that's not physics. If you want a physical picture for where electroweak symmetry breaking comes from, you need to find some principles that allow you to compute the Higgs potential. That is what we mean by having a physics explanation. And, as I said, we know that there is no such physics explanation within the Standard Model. Within the Standard Model, the parameters are all technically “renormalizable” couplings. This means that you just have to put them in by hand. Whatever they turn out to be, you have no control over it. You just adjust them from experiment. You can make a lot of great predictions that way, once you know what the parameters are. But you have no idea where the parameters come from. To compute the parameters, you need a deeper underlying theory. And, as I told you already once in this interview, the underlying theory has to involve some new physical forces that we have not yet discovered. Those new forces are out there, and it is just imperative that we find them. And to make a theory of the unification of forces without knowing what those new forces are, that makes no sense. You've only got part of the picture.

Zierler:

What is the lack of what's been seen since 2012 at the LHC, searching beyond the Higgs? How might that buttress your feelings about all of these things?

Peskin:

That's a whole discussion. Let's wait until we get there.

Zierler:

On a slightly different topic, tell me about your interactions with Daniel Schroeder leading to the textbook Introduction to Quantum Field Theory. What were the origins of that partnership?

Peskin:

It's not a long story. Dan was a student when I gave the course for the first time at Stanford. Actually, I was doing a lot of teaching there. I guess for the past maybe 15 years, I've been teaching one quarter a year. But at that time, I was teaching more or less full-time. The Stanford Physics Department was not strong in teaching, at least in the advanced courses. More recently, they have been doing much better at the undergraduate level. Patricia Burchat is a fabulous teacher, and, recently, Carl Weiman joined our faculty.

Zierler:

He cares a lot about pedagogy.

Peskin:

Well, not only does he care about it, he's one of these people who's an expert, who does experiments on how you do pedagogy well for beginning students. But for the graduate program, people just weren't very interested in it. And so, there was a vacuum to fill, and I like teaching, so I was doing a lot of teaching. I taught statistical mechanics, and then I taught quantum field theory, taking my old notes from Cornell, and stretching them out, and enhancing them to a full one-year course in quantum theory, three quarters. Dan was a student in both of those courses. I'll tell you an anecdote about this. One day, in the statistical mechanics course, I came in, and there's something taped to the blackboard. It was a parody of the Major General’s song from the Pirates of Penzance. One of the lines was, "And now, I'll show you something that will tell you all how sly I am—the exponentiation of the disconnected diagrams." This theorem appears in the proof of the cluster expansion in statistical mechanics. It is a very beautiful theoretical trick. I am sure, though, that it went totally over the heads of 90% of the students when I presented this, and so it made a great joke. That's how I met Dan. He was one of those people, very unusual among Stanford graduate students, whose ambition was not to be a great researcher and win a Nobel Prize, but rather to be a liberal arts college teacher. He was very serious about teaching, great at explaining things. After I taught the course twice, I had a big pile of lecture notes. Also, at that time, there was a vacuum in terms of quantum field theory texts. Bjorken and Drell was out of date after the gauge theory revolution of the 1970’s and the discovery of the power of the renormalization group. There was nothing comparable that filled that space. So I proposed, “The book needs to be written. Dan, this would be a great thing for you. Why don't you just type up these notes, we'll be done in a year, and you'll have a credential when you go to apply to be a professor at a liberal arts college."

Zierler:

Besides a credential, though, there needs to be some recognition that there's a gap in the literature, that there isn't a textbook that's good enough and is already doing what you proposed to do.

Peskin:

That is what I just said. It was well-recognized that there was such a gap in the literature. Bjorken and Drell was a great book for its time, which was the 60’s. But many new developments of the 70s were now playing an essential role, including gauge theories, the epsilon expansion, and the renormalization group.

Zierler:

So this isn't so much a gap, this is a much needed update.

Peskin:

Yes. Some books had been written to fill that gap. My colleagues at Saclay, Claude Itzykson and Jean-Bernard Zuber, wrote a very nice textbook. But I taught out of that when I was at Cornell, and my students absolutely did not like it. Too much French rigor, I think. There were other books out there that were also not successful. And so, really, there was a big vacuum, and a new textbook was needed. I thought, "I have all these lecture notes. It's got everything you want. I've got this great guy who really understands what an explanation is, is a great pedagogue, he can help me write it up, we'll publish this, and we'll be done in a year." But you know how these things work. Eight years later, we final published the book. By that time, Dan was already a professor. But that book has been tremendously successful. It's still, I think, the leading textbook on quantum field theory. And when I travel all over the world, students want to meet me because they've used that book. It's very pleasant, actually, to feel that all of that work produced something useful.

Zierler:

We've talked about the early years. We have to talk about the aftermath of the collapse of the SSC. What was lost? Let's talk specifically about, for example, supersymmetry. What may have been exciting before 1993 that remains stuck as a result?

Peskin:

I think the big loss is really in terms of the prestige of the US high energy physics community. Frankly, we never recovered from the SSC going down. Actually, I saw an interesting curve some time ago of the total US funding level for science. There actually is a visible inflection point when the SSC was canceled. If you just look at the total US funding for science in real dollars, you can see the inflection point there.

Zierler:

Well, in the case just how the SSC was, too.

Peskin:

Before the SSC, there was a big internecine argument between the high energy physicists and the condensed matter physicists. The condensed matter folks said, "The total funding for physics is probably capped, but if you transferred it from high energy physics to condensed matter physics, it would be so much more useful, and we'd progress so much further. Who needs these huge experiments with thousands of collaborators? It's all about individual people in the lab who are making the big discoveries." Every part of that statement turned out to be wrong. Actually, the funding for condensed matter physics also went down when the SSC was cut. The whole area of physics lost prestige. And neither area has really recovered.

At the same time, it's been interesting, over the past decade, to look at what's been going on with materials science and condensed matter at SLAC. People have begun to regard synchrotrons and neutron sources as necessary tools. Now we also have X-ray free electron lasers that are proliferating around the world. People are just lined up to use those machines. At an X-ray laser, the experiments are done by collaborations of 100 people. The experiments are no longer done in an individual laboratory. You need a big team to work with these facilities. So gradually, I think, the condensed matter and materials science world has been turned around to the power of Big Science. But unfortunately, they didn't understand this then, and we didn't know how to explain it well enough. So it was just a disaster. And it may actually be irrecoverable. I don't know. Certainly, in high energy physics, it's not recoverable without a big, unexpected discovery.

Zierler:

I know this is forward in the narrative, but given the way you lament the loss of this, what are your earliest thoughts about what ultimately would become discussions related to the ILC?

Peskin:

I've been working on the ILC in some aspect for a long time. I already told you that I was one of the authors of a 1988 report about physics at future e+e- colliders. In 1989, when the LEP experiments and SLC started, people were asking, "What's next?" So in October 1989, I gave a talk at SLAC about the next stage of e+e- collider physics, where you would go up to 400 or 500 GeV, you'd study the Higgs boson, you'd study the W bosons, maybe you'd discover the top quark, and you'd study that quark in great detail. Maybe you'd discover supersymmetry with that machine. There was a whole menu of measurements that were obvious to do. This was 1989. I found out later that, completely independently, Kaoru Hagiwara had given that essentially the same talk at KEK. And Peter Zerwas had given the same talk at DESY.

Programs were independently launched at these three places, and in Novosibirsk, to try to develop the next-generation e+e- collider at 500 GeV to 1 TeV. In the fall of 1990, these activities got hooked up, and an international community was formed. I had a wonderful experience in the fall of 1990 attending the third JLC (Japan Linear Collider) Workshop. I was one of two speakers who gave their talks in English. At that time, I'd never studied Japanese. But I knew the subject of e+e- physics. So I could look at the slides and figure out what was going on. I made some friends there who are still my close colleagues today. It was a great experience. And then, we held the first international conference on linear colliders in 1991 in a very remote place in Finland. (The story was that, according to Carlo Rubbia, the meeting had to be in Europe but as far from CERN as possible.) So, this community has been around for a long time, but its fortunes have continually been going up and down, with a lot of the particle physics community being very skeptical about it.

Over years of study, we learned a lot. We understood how to do the precision measurements of the Higgs boson. The first major paper on this topic was that of Barklow, Burke, and Hildreth, referred to above. It was published in 1994, but much of the work was done for our 1988 report. We learned a huge amount about how to measure the properties of supersymmetric particles. One of my big activities in the 90’s was thinking about how, if you discovered supersymmetry, you could measure the full set of the parameters in an e+e- collider. Supersymmetric models are very complicated, and you can't really understand them unless you specify the parameters, so you have to figure out how to learn their values experimentally. That turned out to be a very interesting study. With Hitoshi Murayama, Xerxes Tata, and my student Jonathan Feng, we wrote a paper on how to test the symmetry relations of supersymmetry. For example, can you prove by measurement that the photon-W-W coupling is equal to the photino-wino-W coupling? This was all fascinating, and I think it fed into the general discussion of trying to understand supersymmetry experimentally. But in practical terms, the fate of these e+e- colliders has continued to go up and down.

The top quark was discovered in 1995. Soon after this was Snowmass 1996. We at SLAC pushed for a domestic e+e- collider at that meeting. But in the end, we lost out to the idea of a muon collider. Robert Palmer gave a very impressive talk about the possibility of a muon collider. He claimed it could be built for a billion dollars. This turned out to be totally incorrect. But with all of the political arguments in the community at that time—-the preparation for the Tevatron run 2, the preparation for the LHC, the idea that ultimately the muon collider was coming, and the idea of our relatively small group that the e+e- collider was the right way to go—-our community didn't have a consensus and couldn't really get any traction. Five years later, this was repeated. In 2001, there was a Snowmass meeting. This was followed by a committee headed by John Bagger and Barry Barish that recommended a domestic e+e- collider at Fermilab. But the community did not gather behind this idea. Eventually, that was deemed too expensive, and Fermilab dropped it like a hot potato.

Now there is some momentum behind the International Linear Collider, the ILC. This is a mature proposal for an e+e- collider developed by a global collaboration. The Japanese have shown interest in hosting it. But there is a lot of skepticism about whether that's real and whether actual money is going to flow into this project. I'm hoping we're going to find a positive sign in the next year, but we don't know. On the other hand, no other project has materialized in high energy physics that can realistically be built in the period just after the LHC. I think if you were a betting man, you would bet that LHC will be the last great particle accelerator, and the whole field will dissipate after the LHC is over.

Zierler:

This returns us to our previous question about your disagreement with looking at the Higgs as the capstone to the Standard Model.

Peskin:

Because I think that that's just the world's stupidest idea, that you stop after the LHC when there are strong indications that genuinely new forces of nature are out there to be discovered. But I don't happen to have $10 billion in my pocket. So you have to argue this out intellectually in some way.

I do have great hopes for the idea that the ILC can be built in Japan. It would be a big stimulus for the northern Tohoku region, which was devastated since the 2011 earthquake and still has not fully recovered. There are more domestic motivations for putting money into a global science project in the north region of Japan, the Tokyo region. Japan has a declining economy. The Japanese have a problem with demographics, the age distribution, how they're going to support themselves in the future. Japan has very few natural resources. They've got to have a plan. And they're not doing very well at it. And so, it's some people see hosting a global science project an ingredient that would help them along the direction they ought to go. But is that enough? Other people in Japan see it as an expensive distraction. This is true particularly for with scientists in other fields, similarly to what we saw with the SSC. We still haven't found out who's going to win.

Zierler:

To go back to one of my earlier questions about the trend for particle theorists becoming more involved in cosmology and astrophysics, at this point in the narrative, in the late 1990s, early 2000s, I'm curious about your contributions to the search for dark matter.

Peskin:

Well, they're pretty small, so that's fine.

Zierler:

How did you get involved?

Peskin:

I got involved through ILC, actually. In the period in the early 2000s, when ILC was in the ascendant, and we were talking about having the ILC sited in the US, at Fermilab. The question was asked: What can ILC do for cosmology? And this is a question that I had some tools for because I'd been spending all of this time thinking about how you determine parameters in supersymmetry. If you have a supersymmetric model of dark matter, which was the favored model at that time, then dark matter would be a weakly interacting massive particle, a WIMP, for which the supersymmetric partner of the photon would be the natural candidate. Then, if you discovered supersymmetry at the LHC, you could go with an e+e- collider and measure its properties precisely. And after that, you could use this information to pin down the nature of dark matter. It would be possible to prove that dark matter really did come from supersymmetry. You would be able to compute from first principles the amount of dark matter that there should be in the universe, and, with luck, the result would agree with the value that is measured. And there are other things you could compute, such as direct detection cross sections, and those would also be testable predictions.

I got very excited about this. And so, a group of us, with Ted Baltz, a post-doc at KIPAC at that time who was probably the strongest member of our collaboration, Marco Battaglia, and my student Tomer Wizansky, we wrote a big paper, almost 60 pages in the Physical Review, in which we studied explicit models, and we looked at how well they could be measured and how well you could predict all these things. And in its time, I think that was quite influential. Today, the sort of models that we studied are no longer considered good candidates for the explanation of dark matter. Many of the particles required in these models are now excluded by searches as the LHC. So we have to rethink things. But I still feel that the general idea remains correct. We need to discover new particles in accelerator experiments, measure their properties with high precision, and then predict quantities that can be compared to measurements from astrophysics.

The current trend in particle physics is that people have started thinking about that maybe dark matter is not supersymmetry at all, it's a particle in some weakly coupled so-called “dark sector”. It is possible to write theories of a dark sector. It would be connected to the Standard Model by specific interactions called portals. And these interactions have very small coefficients, which is why dark matter has weak interactions with the Standard Model. This is a whole new area of theoretical endeavor.

One thing that's very cool about the dark sector story is that you can test these new kinds of models at low energy accelerators by just getting a lot more statistics and looking at the results with higher precision. There are a number of initiatives around the world to do that, many of them at the Jefferson National Laboratory. The trend was started by a group of SLAC post-docs who were working on this. Actually, two of them are now back at SLAC on our faculty, Philip Schuster and Natalia Toro. There was a very influential paper written by Schuster, Toro, Rouven Essig, who's now at Stony Brook, and Bj Bjorken. I got the postdocs connected to Bjorken, who, in addition to his theory work, had designed and run experiments looking for very weakly coupled particles. The four of them wrote a paper that said, "Turn on those low energy electron fixed target experiments again, and discover this new sector."

It's turned into a big enterprise. We're mounting an experiment at SLAC in that area called LDMX, which oddly, is parasitic on the next generation free electron laser LCLS II. And all of my young theory colleagues are now working on this stuff. You can invent a new model of light particles with a weak coupling to the Standard Model through one of the portals, and then there are low energy experiments by which you can discover or exclude this model. Once you have written that paper, you encourage people to do the experiment. It is a great new development. But all these experiments could come back negative, and the whole trend will evaporate. So I don’t know what the outcome will be. But at the moment, this is probably the hottest topic in phenomenological particle physics.

Zierler:

I know you've talked about this before, what are your reactions to the term “God particle”?

Peskin:

I hate it. But I'm supposed to not hate it. Some years ago, Alan Alda became interested in the fact that scientists have such difficulty communicating with the public. So he gave workshops around the country, basically, teaching scientists how to improvise and how to communicate. It was very interesting and fun. Alda held one of these workshops at Stanford in 2014, and I got myself invited to it. And I asked him this question. "What am I supposed to do with this term, the God particle? It makes no sense." His response was, "If it gets the attention of the public, you just have to work with it. You shouldn't shun it. That's the worst thing you could possibly do." That was good advice.

Zierler:

Yes. But for you, I wonder if your reaction is even more visceral because of your views on the Standard Model and what the Higgs does and does not contribute to it. In other words, the God particle suggests that it's the thing that explains it all, right?

Peskin:

Yes. I think that's the very odd aspect of invoking the divinity of the Higgs boson. It's a little too much for me. But yes, according to Alan Alda, I have to work with it.

Zierler:

I'll return to my earlier question about your other major textbook, Concepts of Elementary Particle Physics. Was this also the case where the literature needed to be updated, or from a pedagogical perspective, you really sensed that there was a need for a new or different approach?

Peskin:

This is a brand new book. Just now, it’s not clear how well it's going to do. The response to the quantum field theory book was great. Everybody could see that there was a vacuum, and so people took up that book immediately. In fact, Dan and I circulated the quantum field theory book in manuscript, actually, even before you could put things on the Web. And people were asking for more. For the new book, I know that there are a number of people whom I know who think it's a very useful book. But the judgment of the community is still out. So we'll see how it does. I think that there is a vacuum in the following sense: It's very easy to write a textbook about what the ingredients of the Standard Model are, and then you can say, "Well, there are these things called Feynman diagrams, and here are the rules, so you can compute some cross sections." And then, look, they agree with experiment. So one can explain at that level what the Standard Model is and why we think it is correct.

The Standard Model, though, is deep, and it has a lot of subtlety. And I felt that there was no textbook that allowed students to understand the Standard Model deeply and that one should be written. This is especially important for experimenters who cannot sit through a whole year of quantum field theory. I give a lot of lectures at summer schools, lectures for graduate students about various properties of the Standard Model. And in particular, since the 90’s, I've been lecturing a lot on the weak interactions, discussing why what was measured in the precision electroweak program was very nontrivial, and about the structure of the Standard Model. I felt that all of that really needed to be explained in a textbook book. Most of the material was in lecture notes I've given in various places, so all I had to do was organize that. I taught the course three years running at Stanford. And in that time, I was able to write up this material properly. Also, you'll see, I've been collecting things like event displays, and graphs for which, basically, you can do a calculation in the textbook, and show the figure in which the curves look exactly like the result of that calculation. My whole career, I've been collecting things like that. So it was good to just put it out there. If you read this book, you will really understand why the Standard Model is true. You’d be convinced. So I hope it will catch on. Perhaps some students will not want to understand the Standard Model at that level, which I think would be a shame. But they should.

Zierler:

As your lectures and writings have indicated, there's still much mystery surrounding the top quark. I wonder if you could talk a little bit about that.

Peskin:

This is a very interesting topic. There are two views of the top quark. To introduce these views, we need to talk about quark masses. Quarks have a mass spectrum, and the mass spectrum comes from the Yukawa couplings. Yukawa couplings are the couplings of quarks and leptons to the Higgs boson. Today, we don't know how to compute these Yukawa couplings. That's something else that we expect would come from a true underlying theory of electroweak symmetry breaking. For the moment, they're just numbers. We measure the masses, we divide by the Higgs expectation value, and the ratios gives the values of the Yukawa couplings. Using those values, you can compute the rates of Higgs boson production and decay reactions. Now that we've discovered the Higgs boson, we can test those predictions. The predictions agree with experimental results from the LHC at the 10 to 20% level. This is one of the biggest achievements of the LHC experiments.

For the top quark, the Yukawa coupling is actually equal to 1. For all of the other quarks and leptons, the Yukawa couplings are small numbers. So you can interpret that in one of two ways. One is that the top quark satisfies the simplest expectation. It's the most ordinary quark. All the other Yukawa couplings are suppressed for some reason, and we need to find the reason for that suppression. The other point of view is that the top quark, being the particle of the Standard Model that's most strongly coupled to the Higgs boson, is the key to electroweak symmetry breaking. And so, the theory of electroweak symmetry breaking necessarily makes special use of the top quark. In this point of view, the top quark can have strong interactions with the Higgs boson and can even be composite. And we don't know which is right. Supersymmetry actually is an intermediate case. The mechanism for electroweak symmetry breaking in supersymmetry usually involves the top quark, but this is just one among many contributions. It's the biggest one, and it gives the right sign, that the symmetric vacuum state is unstable. But the top quark doesn't have any special structure.

On the other hand, there are other kinds of models. One class of these is called the Randall-Sundrum models. These models have an extra (5th) dimension. The top quark is part of the calculation of electroweak symmetry breaking in a much more intrinsic sense, and also the top quark becomes, these people say, partially composite. So it has, itself, composite structure that, in principle, you can demonstrate by doing experiments in which you measure the weak interaction couplings of the top quark. Today, we don't know the answer to that question. But it is tightly bound up with this other question that I asked you, what is ultimately the mechanism of electroweak symmetry breaking?

Zierler:

Ten-plus years out, I wonder what was exciting about the first muons at the LHC, and what was underwhelming in retrospect?

Peskin:

You're looking at that little paper I wrote called The First Muons at the LHC?

Zierler:

Yes.

Peskin:

I imagined somehow that I would write a series of articles aimed at undergraduates about the discoveries of the LHC. But this lost momentum. I'm sorry.

Zierler:

That's too bad.

Peskin:

I wrote two of them, but then the project lost momentum.

Zierler:

What were you envisioning? What would've been the broader project?

Peskin:

We were all hoping that new particles would be discovered at the LHC. And then, you could talk about how they were discovered and what we learned from them. I think, really, the study of the Higgs boson could deserve several more of those papers. I just never got around to writing them. I'm sorry. But the story of the first muons was the first physics result from the LHC. Very early, it was possible to measure the mass spectrum of muon pairs. And what's very cool is that it recapitulates the history of particle physics. So you can see the rho and omega resonances, the J/psi, the Upsilon, and the Z. And it gave hope that there would be something else on that spectrum that hasn't yet been discovered. And so, it connected the LHC, in a very visceral way, as the successor to a long history of particle physics. Also, the fact that you could make those curves so beautifully proved the detectors worked amazingly well. And that was quite nontrivial.

Zierler:

This is a question that's not rooted particularly in the chronology, but we haven't yet talked about, as professor at SLAC, when you've taken advantage of that courtesy opportunity to teach in the physics department, serve on graduate committees, or work with graduate students generally at the lab.

Peskin:

Well, constantly. If you're at Stanford, you get some of the best graduate students in the country coming through your department. Obviously, you want to work with those people. So that's been a big thing in my career. I don't know how many of my students you know. We talked already about Emil Martinec. He is now a professor at the University of Chicago and was briefly the director of the Enrico Fermi Institute. Probably my most famous student right now is Jonathan Feng, who is the department chairman at UC Irvine. He is a major expert on dark matter. He is one of the people that I worked with in the 90’s on the measurement of supersymmetry parameters. That's how he got his start. Then there is Matthew Strassler. He has had a very strange career. But we worked together on the first paper on the top quark threshold at e+e- colliders, and that was a great experience. There's a fellow named Michael Mattis, who has done a lot of very beautiful work on the physics of solitons in quantum field theory.

Another former student who is now well-known now is Maxim Perelstein. He is now a professor at Cornell. He was actually a student of my colleague Lance Dixon. His thesis was on high-order perturbation theory of gravity, but he moonlighted with me on the study of the phenomenology of models with extra dimensions. In the late 90’s, we wrote some of the first papers on that subject. It's incredibly exciting to work with these people, and it's been a major part of my career. Also, every year, more or less, I've been teaching at Stanford. I've taught quantum field theory, statistical mechanics, and undergraduate quantum mechanics. I think that last course was not very good. My approach was a little too sophisticated, even for Stanford undergraduates. But I enjoyed teaching that material. Teaching fundamental quantum mechanics for the first time — that's always a great experience.

I've taught classical mechanics and fluid mechanics. When we started the KIPAC astrophysics Institute, it was obvious that a class on fluid mechanics was needed. And you probably know Risa Wechsler. She's now the director of KIPAC. I told her, "I know something about fluid mechanics because I took this course from George Carrier." I joked, "That must've been before you were born." It turned out that I actually did take the course before she was born. But it gave me enough background to struggle through teaching that course a couple times. I think the students got something out of it. I learned a lot. I find it intellectually invigorating to teach courses. And if you do it well, the students come out with knowledge they wouldn't have otherwise had. You always try to do more than what's in the textbooks. So it's a good experience for everyone.

Zierler:

Another very broad question, your tenure at SLAC is long enough where you've seen how it's changed institutionally. Would you say that the core physics mission of SLAC is fundamentally different than when you started or not?

Peskin:

Yes, certainly. Of course, there was a major change in the mid-2000’s, when the BaBar experiment ended. That was the last experiment, the last major high energy physics collaboration, that was based at the SLAC accelerator. To follow BaBar, there was the vision to build a free electron laser. And that has been a sea change in the whole organization of the laboratory. The idea for the X-ray free electron laser was originally put forward by Claudio Pellegrini. He was very far ahead of where the condensed matter community was at that time.

With the end of the BaBar experiment, high energy physics would move to higher-energy accelerators remote from SLAC. Then the dominant player in the organization of the laboratory would be the DOE Office of Basic Energy Sciences, rather than High Energy Physics. That has been an enormous change for everyone. I think, by now, most of the high administrators at SLAC don't have any memory of that change. But certainly, it was a big deal. And it changed a lot of things in the perspective of the laboratory.

But the time that I came to SLAC, it was also a transition time. Because in the 1970s, SLAC was so successful. First of all, they'd discovered the deep inelastic scattering. So they'd discovered the quarks. This result led to the discovery of asymptotic freedom, which is the key to QCD and the theory of the strong interactions. In parallel with Brookhaven, they discovered the J/psi. But then, because SLAC had an e+e- collider, they could work out the whole spectrum of charmonium. It was an extremely deep experimental study. And then, they did the experiment that discovered parity violation in deep inelastic scattering, which was the keystone of the proof that the Standard Model of weak interactions was correct. All that that happened before I came to SLAC, mostly when I was a graduate student.

So by the time I came to SLAC in 1982, the very important times for the lab were over. We had the PEP accelerator and there was a competitor PETRA at DESY in Germany. Those accelerators didn't excite people as much as the discovery of the W and the Z at CERN. And then, there was this move to the supercollider, which SLAC did not participate in. At SLAC, we were working toward the SLC, which I think was a very important machine for the future, but it had a lot of teething problems. It was competition with LEP, and was not up to the competition. So the experimental particle physics at SLAC suffered a big decline in prestige from the 70’s. Still, as a theorist, you have to keep theory going. I think theory at SLAC continues to be very strong. But I watched as the big news in high energy physics went to other places, and SLAC became a much smaller participant.

Still, I think that the skill of our experimenters is really something that's very impressive to me. What was done with BaBar, if you look into how those experiments were done, it was really fantastic. A lot of new experimental concepts were developed during that time, leading to much more sophisticated data analysis.

The contribution that we've made to ATLAS is very impressive. I think it's not an accident that in the Higgs discovery seminar that Fabiola Gianotti gave, there were a bunch of SLAC people who were on the feature slide, even though they actually didn't work on the Higgs boson. These were the people who really figured out how to do precision calorimetry and something called “pile-up mitigation”, which is a very important issue at LHC. The SLAC group realized very early would be an issue and figured out how to solve the problem. But our experimental group is getting smaller as people retire. The future of it is very much in doubt. We'll see how it goes. I think if ILC happens, we will play a major role. If ILC doesn't happen, we'll just participate in the general decline.

Zierler:

To bring your earlier interests in string theory up to date, where is that field interesting to you currently? Where are things happening that deserve patience because of some future breakthrough or testability? Are you impatient with string theory? And how has it changed since you first got involved?

Peskin:

There are two aspects to that. The first is, what do you think the goal of string theory is? And the second is, how much of it is the very high mathematics as opposed to the kind of mathematics that ordinary theoretical physicists know? So let's start with that second topic. I think one thing that happened in the revolution of 1984 was the realization that some truly high level mathematics was involved in string theory. There were signs of this before. Peter Goddard and other people had discovered things about the representations of Lie groups along the following line: "Mathematicians seem to do some of the things that were going on in string theory, so why don't we do exactly what string theory says and see if we can find better solutions?" This intuition turned out to be correct. Eventually, there were conjectures in algebraic geometry that you could test by doing constructions in string theory. This has actually been a very important development in the field, this linkage between string theory and abstract mathematics, algebraic geometry, the theory of infinite dimensional groups, and topics like that.

This is something that I decided that I had no talent for, and so I never went along that direction. Nowadays, people still write a lot of papers about, "Here's a conjecture about this geometry, and we can test it by making a string construction." And yes, it works. It is interesting that some of these results link up with some of the more physics things that you would want to do out of string theory. But it makes some of these results inaccessible to someone who has poor mathematics like me.

At the same time, the goal of string theory has evolved. We are still trying to figure out what problems strong theory is supposed to solve. So let me make a list of possibilities. The original motivation for string theory was that it should give the S-matrix theory of the strong interactions. This is what motivated Veneziano, Koba, and Nielsen. I talked about that earlier in this interview. That turned out to be not the right way to look at the strong interactions. In fact, people who were interested in strong interactions abandoned string theory when QCD was discovered. After this came the idea of Joel Scherk and John Schwarz that you should really use string theory to build a theory of gravity. This goal of string theory is certainly something that string theorists appreciate, but I think it's very under-appreciated in the discussion of string theory by the general public. The model that string theory gives for the theory of quantum gravity is a much more sophisticated one than any other theory of quantum gravity that we have at the moment.

Zierler:

That doesn't mean that it's correct, it just means that it's the best one that we have.

Peskin:

I would say that it is the best, but it’s also more sophisticated in a certain way. What's the problem with quantum gravity? As long as you can do perturbation theory, the problem with quantum gravity is just that it's non-renormalizable. But we know how to calculate non-renormalizable models. You just add a new parameter for each new divergent amplitude. And everything beyond those parameters is a prediction. So, for example, a whole theory of low energy pion physics, which uses a non-renormalizable Lagrangian, but still it makes definite predictions that are verified by experiment. In gravity, you can build up the quantum theory in the same way in perturbation theory. There are problems because the perturbation theory is very complicated, and there are interesting ideas about how to simplify those computations. But in principle, the perturbation theory can be carried out systematically, and to arbitrarily high order.

One of the nice things about string theory is that resolves the divergences in gravitational perturbation theory. The infinities are made finite in a very unexpected and counterintuitive way. This is a contrast to other theories of quantum gravity such as theories with a minimal length, where the infinities are cut off in a more obvious and crude way. In string theory, there is a sense in which there is no distance shorter than the string scale. These short distances turn out not to exist. If you try to go there, you end up in a copy of another geometry in which the distances that are you studying are much larger. So string theory allows us to do quantum perturbation theory calculations within string theory without meeting explicit infinities. This contrasts very much with other proposals for quantum gravity.

I should probably also add that there's an interesting new direction where people who have been working for decades on how to do higher loop computations in QCD got interested in quantum gravity and are now doing calculations that support the gravitational wave experiments. These QCD experts are able to go to higher loop diagrams than the professional general relativists can do, because they have all these tricks that come from quantum field theory. It’s a very interesting development. Zvi Bern at UCLA is probably the leader in that area, and my SLAC colleague, Lance Dixon, has contributed a lot also.

More serious problems arise when you talk about gravity in the strong or non-perturbative regime. Classically, we know that the singularity at the center of a black hole is unavoidable. Any modification of the equations of gravity, more or less any reasonable one, leads to a singularity. How are you supposed to treat that? A part of this problem is the black hole information problem. If you put information into a black hole, the quantum state has to be able to be diverse enough to reflect that information. If you have a unitary evolution in quantum mechanics, when the black hole evaporates by Hawking radiation, the information has to come out in the emitted radiation. But there's the “no-hair theorem” that says that a black hole can have only a small number of variables. So is the information lost when it falls into a black hole? This is a problem that Stephen Hawking started debating in the early 80’s, and there's been a long discussion of it. Stephen visited Stanford in 1983 and, after an interesting set of discussions, Lenny Susskind, Tom Banks, and I wrote a paper proving that his viewpoint was wrong. But we could not propose the right answer. Then, about 25 years ago, Gerard 't Hooft and Lenny Susskind came up with a paradigm where there's a membrane that surrounds the event horizon that can carry many degrees of freedom. This membrane can hold the information, and eventually can give it up. It turns out that this paradigm is actually realized in string theory. Andrew Strominger and Cumrun Vafa and then Juan Maldacena pointed this out in the late 1990’s.

That kind of question gets to the mystery of what quantum gravity is about. What really happens at short distances? What happens near singularities? Is quantum mechanics still correct there? Is there a conflict? My personal prejudice is that quantum mechanics has to be correct, and quantum gravity has to work itself out to be consistent with that.

That's one of the things that makes string theory really special. Recently, in string theory, there's been a big interest in this area of the black hole information problem. What happens to a black hole as it evolves through Hawking radiation? Can we use string theory to somehow count the number of degrees of freedom associated with a black hole? And this is something fantastically interesting, but I've personally gotten further and further away from it. So I probably can't tell you much that's coherent about the answers to these questions. Except that the developments seem to be very important.

String theory also has spin-off applications that have had a large influence One of these is the relation called the AdS/CFT duality. This is a duality between a four-dimensional quantum field theory in flat space, which might have strong interactions, and a five-dimensional theory in anti-de Sitter space. There is a dictionary that takes you back and forth. So you can learn things about four-dimensional strongly coupled theories. Frankly, you often don't know exactly which specific four-dimensional theory you are studying. But there's some four-dimensional theory that you can construct by doing five-dimensional calculations. This idea can be used to make phenomenological theories of electroweak symmetry breaking. I have been working on this recently; we’ll talk about it later.

Also, string theory has applications to the theory of cosmic inflation. My colleague, Eva Silverstein, says, "You can't discuss inflation without discussing quantum gravity because these inflationary potentials have to be so flat that they're affected by quantum gravity perturbations. But fortunately, I'm a string theorist, so I understand quantum gravity. So I can now make consistent models of inflation." And she has carved out a whole niche in which people make explicit string theories of inflation, with predictions can be tested by observations of the cosmic microwave background radiation.

We still have a lot to learn about string theory. We don't yet know what the fundamental equations of string theory are. We don't even know if there are fundamental equations or if that concept is replaced by another concept. People will continue to work on this for a long time, and I think it's going to be very fascinating.

Zierler:

Has there been interesting work in CP violation in recent years for you?

Peskin:

The BaBar and Belle experiments proved that all observed CP violation in particle physics results from the CKM angle. This is a very important development. Before those experiments, there were many alternative theories in play, so that fact that this Standard Model can explain the observed CP violation was actually a big surprise. The result does play into the attitude that the Standard Model is everything and it explains all of particle physics. Beyond the Standard Model, though, there are a million places where you can add further sources of CP violation. Knowing that those are not relevant to CP violation in the weak interactions is a very important statement. Of course, there's always room for some small corrections.

Still, there is a reason to have additional sources of CP violation working at higher energies. It is a mystery where the baryon asymmetry—-the cosmic excess of matter over antimatter—-comes from. This must involve CP violation. But the CP violation in the CKM angle cannot, by itself, explain this. The Standard Model just doesn't have the right ingredients. Inflation, for which the evidence is now very strong, would have destroyed any original matter/antimatter asymmetry and left a situation in which the universe was completely matter/antimatter symmetric. The current asymmetry of matter versus another matter would seem to be easy to develop. It needs only an asymmetry of 10^{-10} in the early universe. But, still, it is not understood. The Standard Model gives a result tens of orders of magnitude smaller. So we need some CP-violating angles that are not present in the Standard Model. These could be in the neutrino sector, or they could be in the Higgs sector, if the Higgs sector is extended, as I've argued to you that it must be. We don't know where this new CP violation is hidden. Of course, it must be insulated from affecting the low-energy weak interactions studied by BaBar and Belle. That is an important constraint.

Zierler:

You mentioned neutrinos just very briefly; we have not talked about then very much yet. And I'm curious what's been interesting to you in neutrinos, given that this is an area, of course, where US leadership is very much front and center.

Peskin:

I just think neutrinos are much less important than most people in my community think they are. I think it is under-appreciated that neutrinos have to eventually get their mass from the Higgs boson. In the Standard Model, neutrinos are massless, but you could make them massive if you just introduce the appropriate Yukawa couplings. In this sense, I feel that the appearance of nonzero neutrino masses is not a proof of new physics beyond the Standard Model. You’d have to have right-handed neutrinos in that case, but why not? We have no evidence against right-handed neutrinos, and all other quarks and leptons have right-handed components.

What that means, though, is that you have to solve the Higgs problem to understand where the neutrino masses come from. And so, that makes neutrinos very secondary in my way of thinking.

Now, it is true that the biggest unexpected discovery in particle physics in recent years has been the discovery of neutrino mass. Neutrino phenomenology has its own interest, and some of the experiments—-in particular, Super-Kamiokande and SNO, are very beautiful. This has become something that you need to know about if you are a particle physicist. But is that telling us something about the ultimate nature of particle physics? I think very little. And so, I'm not an enthusiast of this.

Zierler:

Just to bring our conversation up to the present, the past few years, what have you been working on? What's been most compelling to you?

Peskin:

There are two things that have most of my attention now. They are both connected to the Higgs boson. I've written a bunch of papers with my student, Jongmin Yoon, about electroweak symmetry breaking in Randall-Sundrum models. I’ve told you that it is compelling that there are new particles responsible for the form of the Higgs potential. But now the LHC excludes many models of these particles, at least for new particle masses up to about 1 TeV. This gives rise to a problem that if called the “little hierarchy problem.” If there is a new force of nature that creates the Higgs potential, why is its scale multi-TeV instead of at the Higgs mass scale of multi-hundred GeV? You could say that the idea of this new forces is a total misconception, but if you believe in it, there must be a physical mechanism for this hierarchy of scales. And so, I've been spending some effort searching for that.

The other thing that I've been very involved in is the ILC. As I told you already, I think that there really is a hope that the ILC will be built in Japan. I think there's a certain amount of political support for that that we're seeing now. Not only in Japan, but in the US and Europe. We can talk a little more about that if you want.

I see this as something that is crucial to the future of high energy physics. As I've explained to you, I think there is a physic problem which is very obvious to me, that needs to be solved. This will require data from higher energies. So we have to continue to do accelerator-based high energy physics. But right now, the only thing on the table after LHC is the ILC, or some other “Higgs factory” for the precision study of the Higgs boson. If such an accelerator is not built, we're going to lose the next generation of experimental high energy physicists. And beyond that, there may be no recovery. So this is something I'm taking extremely seriously, and I'm spending a large amount of my time thinking about the organization of the ILC its physics case, and how to promote it.

Zierler:

Where is China in all of this for the ILC?

Peskin:

Let's leave that aside for just a moment. I'll come back to this. Part of this actually, actually, has been very interesting scientifically because there are nontrivial theory questions about how to go from e+e- data to the values of the Higgs boson couplings. The key experimental measurements at the ILC will measure the couplings of the Higgs boson not to 10% but to sub-percent accuracy. At that level, even if this new physics is up at TeV energies, you would expect that it renormalizes the Higgs couplings, and you could measure those effects. Right now, this seems to me the biggest opportunity that exists to prove that there is physics beyond the Standard Model. It’s so obvious. The Higgs boson is key to the whole structure of the Standard Model, and it sits on such a flimsy foundation. We've got to explore this by experiment. But the program needs some theoretical support. How exactly do you go from the measurements to values of the Higgs couplings that can be interpreted by theorists? You have to do something that's more than just calculating some obvious cross sections. You have to understand the phenomenology better. Effective field theory turns out to be a useful tool, so that is also a cool aspect. This is an important thing that I've been working on in the last two years.

Zierler:

To go back to China…

Peskin:

China's a mystery.

Zierler:

You don't see China as a possible host for the ILC?

Peskin:

Actually, the Chinese went off in another direction. A number of people—-Tao Han is very prominent in this group—-put a lot of effort into trying to motivate the Chinese government to host the ILC. But it didn't work. I'm not sure if it was the community skepticism about e+e- or just the idea that, "If Japan is doing it, we should do something different because we should have our own way." In 2013, the Chinese proposed a different method for achieving a Higgs factory, a synchrotron in a large, 100-kilometer ring called CEPC, the Circular Electron Positron Collider. That is an alternative way, which still uses e+e-, to produce a large sample of Higgs bosons that you could then measure with high precision. This is the officially supported direction in China at the moment. Nima Arkani-Hamed, whose name I'm sure you know, is someone who has very actively promoted this. The program would then be similar to the Future Circular Collider (FCC) proposal being put forward at CERN. First you build a large circular e+e- collider, and then later, when you are done with e+e-, you would put a high energy proton synchrotron into the large tunnel and conduct proton-proton collider experiments at 100 TeV. If this is realized in one place or the other, that would be the future direction of experimental particle physics.

In Europe, there's the obvious issue that to build that tunnel in the Geneva region, it would be incredibly expensive. It would cost as much as the LHC just to dig the tunnel before you put anything in it. So that's somewhat daunting. In China, it is likely that this tunnel would be much less expensive. The Chinese are good at large infrastructure projects, and they can choose a region with much more attractive geology. Maybe this large tunnel is something that the Central Committee can just order up. There is a question, though, of how this compares to other options that China has for major science projects. China has an aggressive program of space exploration. Will the CEPC lose out to this, just as, years ago, the SSC lost out to the International Space Station? This is very unclear. The whole political situation is completely opaque. It's just impossible for someone like me to know what the answer's going to be. I think Arkani-Hamed originally hoped that the CEPC would be in the 2016 five-year plan. But now, we know it's not even in the 2021 five-year plan. The next opportunity is 2026.

Zierler:

Is this just a matter of higher energies? It begs the question, why not bigger and more projects within the extant infrastructure at CERN?

Peskin:

Let me say one more thing, and then we'll get to that. The other thing is, the US-China trade discord has become a major factor here. The DOE is now putting up tremendous barriers toward scientific cooperation with China. To fully collaborate with the Chinese on the CEPC is just not politically thinkable at this moment. That’s very disappointing. They're interested in the same physics goals as we are, so we would both benefit from a scientific collaboration. But to have a collaboration for something where you have billions of dollars in expenses seems like a total nonstarter right now. Certainly, this was true under Trump, and probably it continues to be true under Biden as well. I don't know how to get there. There is a serious question of whether Chinese can build this accelerator without Western cooperation. It's not clear. In Japan, they have some of the greatest accelerator physicists in the world. In China, they have a couple of experts, but they do not have the same large and sophisticated accelerator community.

Zierler:

The idea about an LHC 2.0, why is this not the more feasible route?

Peskin:

CERN is definitely going to do the high luminosity LHC. That extends the energy reach, depending on the process you look at, by something like 30%. That's not a trivial step, but it is a smaller step than we had with the original LHC.

To convert the LHC to a higher energy, you would need a whole new magnet development. Currently, the LHC uses niobium-titanium magnets. Carlo Rubbia originally proposed they could be made with fields of 10 Tesla, but the final the LHC design has only 8 Tesla magnets. And they never quite got there. So today they are at 13 TeV rather than 14 TeV in the design. (There are plans to go to 14 TeV for the HL-LHC.) To go beyond that significantly, you'd need to develop magnets that can sustain a higher magnetic field. To do that, you have to change the superconductor.

For FCC, they are thinking about niobium-3-tin magnets, for which they think they expect that they can get 16 Tesla. That would be double the field strength at the LHC. If you put the 16 Tesla magnets in the LHC tunnel, it would be very expensive. There is R&D now on the reduction of the cost. But niobium-3-tin is also difficult to work with, and that adds to the cost of manufacturing. That accelerator would probably cost double what the LHC cost if you built it today. And it would only get you a factor of two in energy. Is that a gamble people want to do? In the recent European Strategy for Particle Physics study, they decided no, they want to go to a much larger energy step. But the large energy step requires building a much larger tunnel because, given the magnetic field, the size of the tunnel defines the accessible energy. For e+e- colliders, the scaling for a circular collider is even worse. Already at the end of LEP at 200 GeV, an electron going around LEP would lose about 1.5% of its energy every time it goes around the ring. It is as if you are not in a synchrotron at all. You have to continually restore the power in order to be at high energy. The power lost to synchrotron radiation goes like the beam energy to the 4th power. So this is a barrier to going to high energy.

So for FCC, they envision a 100-kilometer tunnel, which then dramatically reduces the synchrotron radiation because there's less bending as you go around. But still, they really don't have a decent luminosity beyond about 300 or 350 GeV. You just barely make it to the top quark threshold. And then, beyond that, there's no hope.

All of these considerations imply a very serious problem. With protons and with electrons, we are the very limits of our current accelerator technologies. Something new has to be invented. And those inventions have not been easy to come by. One suggestion is the idea of a muon collider. This would allow you to put muons in a LEP-like ring, and then you could go to 20 TeV, let's say. But the problem of collecting and cooling muons is extremely difficult. I think we're very far from understanding it.

An alternative that some of my other colleagues at SLAC are developing is called plasma wake field acceleration. I think this is very cool. The concept is that, in any case, high energy particles destroy whatever they go through. So you might as well use a medium that's self-destructive, like a plasma. If you shoot an electron beam or laser beam through a plasma, it clears a little, tiny space, about a micron high. All of the electrons in this region get pushed out. This sets up a longitudinal electric field, an accelerating field, and then an electron bunch that is placed just at the right distance behind the lead bunch will get accelerated. The fields that you can get are enormous. The SLAC experiments have demonstrated fields above 50 GeV per meter. Just to set the scale, that is all of SLAC in one meter. If you imagine an accelerator at scale, at 10 or 20 kilometers, you could have an accelerator with tens of TeV. Unfortunately, it is not so simple. The plasma conditions are not easily reproducible shot to shot, there are various instabilities, and the technique deposits energy in the plasma that must be gotten rid of. A plasma cell can only be about meter long, and that can only give you 50 GeV—-or maybe with a more controlled design only 4 or 5 GeV—-and then you have to put that beam into another plasma cell to reach a higher energy. The optics to connect the cells is very complicated.

There different approaches to the design of plasma wakefield accelerators. At SLAC, they are experimenting with beam-driven acceleration, in which the plasma is shaped by a low-energy, high-intensity electron beam. At Lawrence Berkeley laboratory and at DESY, they are experimenting with laser-driven plasma cells. There are a lot of problems to solve.

In all, the situation for future accelerators is quite daunting. Plasma wakefield accelerators are decades away. Muon colliders, similarly, need decade of development. The 16 T magnets needed for the proton colliders are also at least a decade from a practical design that can be built in industry, and it may take decades to persuade anyone to give you the money to build the enormous machine.

All these possibilities are far in the future. We have a serious problem now to find a bridge to the era of multi-10 TeV colliders. What can we propose that is intellectually exciting, that will excite people in the community, that will bring in new students who are going to be eventual leaders in the era when or the very high energy experiments? It is the ILC or another Higgs factory that will fill this gap. And so, as I said, I've been spending a lot of my time recently to promote this idea and get people behind it. There is certainly a core group of people who support this, but it is very unclear what's going to happen. The probability that the result would be a disaster for our community is very high. So it is very much something to worry about.

Zierler:

Well, now that we've approached a crossroads in the narrative, and we've come right up to the present, for the last part of our talk, I'd like to ask two broadly retrospective questions for you and your research, and then we'll end looking to the future. So first, I know it's important for you to communicate your science, your research, to a broader audience, to the public who's interested in these things. Given that ultimately, these projects that are so important for you, these large-scale projects, require public support and tax dollars, what are the most important things for you to convey about your work to that broader audience?

Peskin:

One thing that I actually haven't done in my career is to write a popular book or popular articles about what I've been doing. This is something that's very difficult for me. I have a manuscript for a popular book on extra dimensions, which is sitting in my closet somewhere. There's too much of an impedance mismatch between me and the general public, and I haven’t understood how to solve this problem. There are other people who are very good at it. You probably know of Hitoshi Murayama. He wrote a book on particle physics, which is a bestseller in Japan. Actually, he told me that his contract sent all of the proceeds to the IPMU, his institute in Japan. And then, when he saw how well it sold, he regretted that decision. I once attended one of his lectures in Japan. He's like a rockstar among the younger generation. It's very impressive. But that's not my personality. I don't know how to do that.

I have put a certain amount of effort into the web site usparticlephysics.org, organized by Michael Cooke in the DOE Office of High Energy Physics. Cooke writes communication documents for the public and also for Congress and for the administration. In his group, we debated what are the most important things to communicate what are the key points to put before the public? I think there are a bunch of things that we would like to get across. First of all, particle physics is still a vibrant intellectual activity with big problems, for which the answers are not understood, and therefore big opportunities for discovery in the future. And it may not be so relevant that those don't have immediate practical application because eventually, when we discover new laws of physics, it turns out that we can use them, for better or worse, and we can try for better.

The second thing is that, while are interests center on difficult questions increasing remote from the human scale, to drive to do those experiments leads to new instruments and new technologies.

Indeed, there is a whole line of technology development that has been driven by the development of the particle physics experiments. And that technology development can have important implications. One obvious example is the development of linear accelerators. These are now the workhorses for X-ray lasers, neutron spallation sources, and heavy ion beams. Those facilities are enabling new discoveries in biological structure, materials science, and chemistry. The SLC at SLAC played an important role in making the recent explosions of activity in these fields possible. I joke that one of the problems for the ILC is that, after we developed the needed linear accelerators, they could be used for spin-offs without actually constructing the ILC. But there are new technologies coming for compact and higher-gradient accelerators that will make practical X-ray sources and medical accelerator cheaper and easier to build. I hope that particle physics can get some benefit from these.

Another advance that came from particle physics is the World Wide Web. The World Wide Web was invented by Tim Berners-Lee and Robert Cailliau at CERN. Its immediate application was to replace the system by which, if you wanted to get the data from your experiment to the central computer, you put it on tape and gave it to your graduate student, and he'd ride on his bicycle over to the computer center. Berners-Lee called his system the World-Wide Web when it was running only on two NeXT computers at CERN. His vision was that it would indeed take over the world, and it did.

It is less well appreciated that particle physics has stayed in the forefront of informatics development. This has become more clear in the past few years, with the increasing recognition of the importance of machine learning. Particle physicists have been doing machine learning for decades. The earliest example that I know is the work of Jonathan Dorfan around 1980 to train a machine to automatically discriminate photons from pions in the electromagnetic calorimeter of the Mark II experiment at SLAC. When “deep learning” became popular recently, we found ourselves on the bleeding edge, using this technique for discrimination problems of very high sophistication. With Michael Cooke’s group, we put together a brochure for the public about current progress in artificial intelligence motivated by particle physics.

Particle physics has also driven the creation of unique sensors. These are now considered very important for the development of quantum information. Michael Cooke’s group also has a brochure about this subject. While quantum information itself has played only a small role in particle physics, our needs have contributed a lot of relevant instrumentation. It turns out that there are sensors that particle physicists have been working on for a decade that turn out to be ideal quantum information devices. These are devices that can hold thousands of qubits and devices that can sense individual quantum transitions. We've been developing these for our own reasons. One of the leaders has been Anna Grassellino, who led the ILC accelerator team at Fermilab. Today, when quantum information has become a very important trend in science, this work has turned out to be very relevant.

History shows that the way you make breakthrough progress in technology is to work on problems that are impossible, and solve the problems that stand in your way. In particle physics, the things we're trying to do are basically impossible. but very clever people can solve the problems, and we can make progress. This improves the level of technology for everyone. Of course, particle physics also has the dimension of finding the fundamental laws of nature, which has its own fascination. From both of these points of view, it is very important that the public should understand that we're not finished. Many people give the impression that, "It's the Standard Model. It's everything. You can close the book on particle physics.” But this is just wrong. And I think we have to do what we can to get the correct impression out there.

Zierler:

That gets me to my next question. Of course, a theme in our discussion and your career has been the Standard Model. So I'd like to ask you, as an intellectual construct, what has been useful and essentially unavoidable about the creation of the Standard Model? And then, as a result, what has been problematic, not just for thinking about getting beyond it, but the fact that we even have a Standard Model that, in some ways, perhaps traps us intellectually?

Peskin:

One of the interesting things for me is the origin story of the non-Abelian gauge theories. These were introduced in a paper of C. N. Yang and Robert Mills was written in 1954. But they proposed it as a gauge theory of nuclear isospin, which was not right. In fact, physicists did not understand clearly what the correct applications of gauge theories to particle were until the 1970’s. It is interesting that you can have a mathematical theory that is very beautiful, so it seems that it must have applications in nature, but it might not be clear for a long time what exactly the relevant applications are. Einstein had the intuition that he should study differential geometry, and that turned out to be directly relevant to gravity. Of course, he'd been thinking about gravity for many years before he came to the approach of using higher mathematics. But for Yang-Mills theory, it was the other way around. We found the mathematical structure, but we didn't know how it applied to particle physics.

Always, we needed both theory and experiment, and a lot of false starts, to get to the current picture, in which all of the fundamental interactions are gauge theories. Once you've understood that, it is straightforward to understand most of the structure of the Standard Model. You just need to know what the gauge groups are. The one missing ingredient, as I keep emphasizing, is the mechanism of the spontaneous symmetry breaking. We still don't understand where that comes from or what the reason for it is. A property of the Standard Model is that at short distances, everything is a weak-coupling gauge theory. And so, we haven't had to understand the properties of strong coupling gauge theories, except for QCD. And for QCD, you could say we don't understand it very well. We have lattice gauge theory that gives us the qualitative explanation, and then we can get quantitative predictions by doing big computer simulations. We still don't understand, really, how quark confinement works, even though this problem is a problem, as I told you, that I thought about as a graduate student. We don't understand the answer to that problem in a way that you could explain it to your grandmother today. We know that if you do a lattice gauge theory calculation, it's obvious. And we know that that's somehow continuously connected to the physics that we see in the strong interactions. But to explain that purely within the context of the strong interactions, that’s extremely difficult. So there's a barrier that we haven't crossed, and I keep thinking that we're going to have to cross that barrier to find the right theory of electroweak symmetry breaking. It is very disappointing to me that many members of my community don't think there's a problem there, and do not put this as a direction that we have to go. Because this is the next frontier for the theory of particle physics and the fundamental forces.

Zierler:

That gets me to my last question. Looking to the future, best case scenario, all of your efforts on the ILC, both from supporting the next generation of physicists, and more importantly, for just advancing physics to the next level, best case scenario, what is the timescale that this can realistically happen? And what are you most optimistic in terms of finally breaking beyond the Standard Model?

Peskin:

Well, there are lots of places where you would think there could be progress beyond the Standard Model in the shorter timescale. We do know there's something else out there because of the existence of dark matter. The particle or field that makes up dark matter cannot be a part of the Standard Model. But the explanations for dark matter span from particles of mass 10^{-12} eV to black holes of a solar mass. It is an enormous range. And we don't really know where to look.

We talked a little earlier about low energy dark matter experiments that probe for particles of a few MeV to a few GeV. If one of these would be successful, it would point to a new set of fundamental forces of a kind that I haven't been talking about, which are just not at all related to the Standard Model and very weakly connected. But it would still tell us that there's something else out there. We thought we knew where to look when people thought that dark matter was a WIMP, that it had to be between 100 GeV and a TeV. But the LHC really has dampened that possibility. So when you remove that region, a lot of other explanations, take the fore, and these are spread all over the map. So these low mass dark matter candidates could be discovered relatively soon. Let's say in the next five years. And if one of them were discovered, then that would be a very interesting direction that could be explored.

The big news that is expected in neutrino physics is the discovery of CP violation in the neutrino sector. I think that's very likely to happen. I think also that it is not very likely to give us more clues about fundamental interactions. We have been struggling for decades to understand the origin of the CKM angles, without a finding a compelling model. The neutrino angles have a similar origin. I think we will not understand this until we truly understand the Higgs boson.

There are two things that I'm very interested in in collider physics. One of them is that I think there is still opportunity for particle discovery at the high luminosity LHC. As I say, depending on the particular kind of particle, you get maybe a 20% to 50% enhancement of the mass reach. And we don't understand why the particles that couple to the Higgs boson and create the Higgs potential are as heavy as they are relative to the Higgs boson mass. They could be just around the corner. And if they are, they could be discovered in the high luminosity running, and that would give us a definite direction in which to continue. I am most interested in the continued search for top quark partners, maybe even as various kinds of heavy leptons, that would point toward the more composite-Higgs, strong-interaction type of symmetry breaking theory. We could still find the evidence for this soon. Let's say before 2030, when we see the data from the High Luminosity LHC. This is a direction that's already been explored to a certain extent. The LHC has set limits at about 1.3 TeV. The HL-LHC will move the search up closer to 2 TeV, and hopefully we will discover something along the way.

The big opportunity that I see, I've already said this in this interview, is that we haven't really started exploring the precision study of the Higgs boson. If you make the assumption that the new particles are at TeV masses, that already tells you that their radiative corrections that modify the Higgs boson properties are at the percent level. This means that we wouldn't have expected to have discovered these effects yet at the LHC, and in fact, it's not clear that we can discover these effects even at the HL-LHC. If you need five sigma evidence, and you're making a 4% measurement, a 4% you could prove that the modifications are there only if they are a 20% effect. But we expect that these effects are a few percent. So in some sense, the LHC is hardly even in the game for this.

But you could discover these effects at an e+e- collider. The earliest we could expect an e+e- collider is probably the late 2030s. If you began to build the accelerator and the experiments in the late 2020s, you'll see a motion of people from the LHC experiments onto this new endeavor. You'll see people talking about how important it is to do these precision Higgs measurements. Optimistically, you'll see a lot of graduate students saying, "Yes, this is a great thing to do in my career. We're going to study the Higgs boson incredibly, and we're going to find that it's not the Standard Model Higgs boson. With this, we could really make the discovery.” And then, we'll see where that points us. And, at the same time, if we develop a new accelerator technology, a muon collider, a plasma wake field accelerator, or something else, that will take us into the multi-10 TeV regime. At those energies, the new physics won’t just be a hint. It will be apparent, and we will write a new chapter in the history of physics. So that's what I'm hoping for.

Zierler:

That's a remarkable contrast between where things are now and where they certainly can be headed. So it's exciting.

Peskin:

They really can be headed there. But as a community, we have to wake up and say, "That's the goal. Now it is time to pursue it."

Zierler:

It's been a great pleasure spending this time with you. This has been a remarkably wide-ranging and lucid conversation ranging from the explanation of the science to your vision for where things are headed. So I'm so happy we were able to do this. Thank you so much.

[End]