What drove you into particle physics?

I grew up in an environment that was keen on science. Both of my parents were doctors. My father was a well-known academic surgeon who held appointments at the University of Pennsylvania, the University of Chicago, UC San Diego, and Yale. My mother was a general practitioner. However, my original plan was to study biochemistry and not physics. This field was developing rapidly in the 1960’s — and it still is — so it seemed very promising. I studied rather hard on the route to becoming a biochemist. But it turned out that I was not very good as a laboratory scientist. At that time, to be a biochemist, you needed to be a really good chemist at the lab bench.

But if laboratory work was not for me, I needed to study some more mathematical science. I got interested in condensed matter physics and fluid dynamics. This was just at the time when Ken Wilson introduced his revolutionary ideas about phase transitions, for which he later won the Nobel Prize. I got very interested in that and had the ambition to go to Cornell to study with him.

I arrived at Cornell in the fall of 1973. Asymptotic freedom had just been discovered,  and Ken had just invented lattice gauge theory. He told me that he had finished with phase transitions. “It's time to solve the strong interactions.” So that is how I became a particle physicist.

Which were the open questions that you faced as a graduate student?

As a graduate student, I wanted to maintain very broad interests. When I came to Cornell, I didn't know whether I was going to do particle physics or condensed matter physics. Both were fascinating to me, so, while I was there, I tried to attend both sets of seminars and keep abreast of what was happening in both fields. At that time, I decided that trying to be an expert also in astronomy and astrophysics in addition was too much. And that, it turned out, was a bad decision. Many interesting things were happening in astrophysics at that time. The things that made the biggest news were the discoveries of 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 long time to catch up to the importance of this fact for particle physics.

In particle physics, the question that I spent the most time on was quark confinement. At that time, it was clear that quarks were permanently confined in hadrons, but there was no simple explanation of the mechanism behind this. To a certain extent, this is still a problem today. We know from lattice gauge theory that quark confinement is absolute, but we still lack a concise description of the physics. It is said that you don’t understand something until you can explain it to your grandmother, and this is still definitely out of her reach.

The 1970’s was a very exciting time to be a graduate student. The J/ψ was discovered in 1974. Suddenly, all of the skeptics became believers in the quark model. In parallel, there were important developments in theoretical physics, the discovery of classical solutions and instantons. At that time, it was fashionable to think about chiral symmetry from current algebra, but I studied it from a more mechanical point of view. I learned about the Nambu-Jona-Lasinio model, which was very much out of fashion. Yoichiro Nambu and Giovanni Jona-Lasinio, back in 1961, had an explicit mechanical approach to chiral symmetry breaking based on the theory of superconductivity. Their paper was under-appreciated for a long time, but it is a path-breaking paper that is still worth studying. 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.

In your view, when did the Standard Model appear as a theory?

As soon as ’t Hooft and Veltman proved that gauge theories were renormalizable, theorists began to believe that these theories explained the strong and weak interactions. One of the most enlightening moments in my graduate career came in the aftermath of the was ICHEP conference in London in 1974. By then, all of the ingredients of the Standard Model were in place: Asymptotic freedom was already established, we had an SU(2) x U(1) model for the electroweak theory and even the SU(5) grand unification. During that conference, Jean Iliopoulos laid out with great accuracy what we now call the Standard Model.

Ken Wilson attended the conference and, after he returned to Cornell, he gave two weeks of lectures on these topics. He covered the big blackboard in the Wilson Lab seminar room with boxes corresponding to the open problems in particle physics (deep inelastic scattering, the ΔI = 1/2 rule, etc.). He then started drawing arrows connecting these boxes and explaining how everything fit together in the framework of the Standard Model. The only missing element was the charm quark. But this was discovered a few months later, offering the final piece in this picture.

Already at that time, many people, including Jean Iliopoulos, Ken Wilson, and Steven Weinberg, believed that we had the fundamental gauge theory of the strong, weak and fundamental interactions. However, it should be said that others, including Richard Feynman, were more skeptical.

Feynman for a long time believed that QCD is not the full story with respect to strong interactions. I met Feynman for the first time at the Orthodox Academy of Crete. I was then a postdoc at Saclay in Paris, and I was invited to present my work on technicolor, an early theory of strongly- interacting composite Higgs fields. I prefaced my first talk by stating that now that the strong and electromagnetic interactions are solved and it was time to move to the next problem. At my second talk, Feynman positioned himself in the front row, and, when I repeated this statement, he ostentatiously stood up and walked out. (He teased me later about his joke.) Feynman was concerned that there are still things in QCD that we needed to understand and I believe he was right. For example, we still do not have the complete theory of hadronization that is needed for precision predictions at the LHC.

When did you start working in the Standard Model?

By 1977, the Standard Model was a well established theory of strong and electroweak interactions. Just as I finished graduate school, the SLAC measurement of parity violation in deep inelastic scattering nailed down the neutral weak current and gave a final proof that the Standard Model of weak interactions was correct. As a junior fellow at Harvard, I saw a discrete change. Before, it seemed that Steven Weinberg and Shelly Glashow were coming up with a new weak interaction gauge group every week. Afterward, they were both writing their Nobel Prize speeches. In this environment, it seemed that it was the right time to think about Beyond the Standard Model Physics. The Standard Model works fantastically well as far as it goes, but it leaves many questions unanswered: What is the cause of electroweak symmetry breaking? What sets the mass of the Higgs boson? Are there additional Higgs bosons, or other particles that interact with the Higgs and explain its properties? I worked on these questions as a postdoc, and I still continue working on them today. Probably my most important contributions are in methods to test for physics beyond the Standard Model and find evidence for models of various types. Unfortunately, up to now, all of these tests have come out negative.

Do you observe a shift in the pendulum between theory and experiment today, compared to the time that you entered physics?

There has been a shift. I believe that it is linked to the establishment of the Standard Model and the level of technical precision that we have reached. When the SppS operated at CERN in the early 1980’s we had only a basic understanding of collider physics. Parton distributions and the factorization formulae were well known. But the practical methods of collider phenomenology were not yet established.

UA1 and UA2, the two main experiments at the accelerator, discovered the W and Z bosons in 1983. The UA1 experiment also claimed the discovery of the top quark and evidence of supersymmetry. However these claims were quickly refuted, thanks to close collaboration between theorists and experimentalists. The claims reflected a lack of understanding of the variety of events that the Standard Model produces at colliders and the ability of the Standard Model to mimic seemingly exotic signatures. The collaboration between theorists and experimentalists continued during the next era of precision measurements of the electroweak interaction. Under the leadership of Guido Altarelli, the theory community fully engaged in calculating one-loop electroweak corrections for the Z boson. This work allowed us to bring the precision of theoretical calculations to the same level as the experimental measurements. In fact, the agreement between theory and experiment is quite remarkable. For the LHC, much attention has been paid to creating very accurate simulations that include the full range of complex, many-body reactions.

However, in the process, each aspect of the work has become professionalized, to the extent that it is difficult for anyone to have a complete overview of the path from data to tests of models. Each aspect— precision calorimetry and flavor tagging, jet reconstruction, precision QCD calculation, exploration of increasingly complex model and parameter spaces—has become a specialized activity that is forbidding to outsiders. We need to at least remain curious about what our colleagues are doing and what problems they worry about. These issues might turn out to be crucial in interpreting any measurement.

A related trend is the growing split between mathematically-oriented and phenomenologically-oriented theorists. Up through the 1980’s, and even after the 1984 breakthroughs in string theory, particle theorists had common seminars in which the full range of topics would be discussed. This is not true any longer. In the quantum gravity community, people even say that we can make progress without thinking about experiments. Of course, the best theorists don’t believe this. Edward Witten wrote one of the first papers on direct detection of dark matter. Juan Maldacena wrote a key paper on the search for non-Gaussian correlations in the cosmic microwave background. And, two of the biggest trends now in mathematical physics, the amplitude program and the conformal bootstrap, originated from questions asked about particle phenomenology.

When did it become apparent to you that we should start working to find physics beyond the Standard Model?

As I said earlier, I started on this already in the late 1970’s when I was a postdoc. Many people were realizing that the Standard Model left important unsolved problems. In 1981, at the Lepton-Photon conference, Lev Okun gave a very pointed talk arguing that the nature of the Higgs boson is the number one problem in particle physics. And, this is still true.

One idea to address this question is that the Higgs boson is not an elementary particle but rather a composite, bound by new strong interactions. Steven Weinberg and Leonard Susskind suggested “technicolor” as a model of a composite Higgs boson. I got interested in this idea and worked on it for several years. This led me into thinking about beyond-Standard-Model physics more generally. When I was a postdoc at Saclay, more people began to think about this, and it seemed that there was a new field waiting to be developed.

In some sense, I've been doing research on beyond--Standard-Model physics continually since that time. The questions that we had at that time are still not answered. Several generations of new models have been proposed and ruled out. Yet, still, I find this compelling. The most compelling aspect, to me, is that the spontaneous breaking of electroweak symmetry is put into the Standard Model in a completely ad hoc way. In condensed matter physics and in chemistry, there are many examples of systems with spontaneous symmetry breaking—magnets, superfluids, liquid crystals. In every case, the phase transition is the result of a compelling physical picture. You don’t just write down a potential and feel satisfied, there is a dynamical explanation. This is even true for chiral symmetry breaking in QCD. But for electroweak symmetry, that explanation is lacking. Ken Wilson taught that quantum field theory is not mystical, it is just quantum mechanics, and it is best understood by applying the methods that we use to solve other quantum mechanical problems. So for the Higgs boson, too, we shouldn’t just write down a scalar field and claim that we are done. There should be a mechanical explanation that must involve new particles and forces not included in the Standard Model. Thus, it implies a new fundamental interaction waiting to be discovered.

When did people start looking for the Higgs boson?

The first papers on searching for the Higgs boson were in the 1970’s. At the 1976 SLAC Summer School, James Bjorken emphasized the Higgs-Z-Z coupling as a method for searching for the Higgs boson. John Ellis, Mary, K. Gaillard, and Dimitri Nanopoulos did a complete study of the Higgs boson decay modes. Boris Ioffe and Valery Khoze discussed the search for the Higgs in e⁺e^- reactions. But to do the search in earnest, we needed higher energy colliders.

A breakthrough in the early 1980’s was the discovery of how to construct general phenomenological models with supersymmetry. It turned out that running the renormalization group equations of supersymmetry naturally generated the negative mass term needed for electroweak symmetry breaking. So this was another possible dynamical mechanism. We hoped that it would be easier to discover supersymmetric particles than to discover the Higgs.

Over the years, additional possibilities were added, such as the Randall-Sundrum model that makes use of an extra (5th) dimension. By the mid-2000s, we had a variety of models of electroweak symmetry breaking. In all of these, the symmetry-breaking potential energy of the Higgs boson was most easily derived from new particles with masses at the weak interaction scale of a few hundred GeV. So this set up great expectations for the LHC.

What have we learned so far from the LHC?

The LHC has done some amazing things. First of all, we discovered the Higgs boson. By now, we have measured many of its couplings with a precision of 10-20% and found agreement with the Standard Model at this level. This is a major milestone.

It is a big step forward conceptually in many ways. In 2010, many of my experimental colleagues did not believe in the Higgs boson, or did not believe enough to orient their thinking around it. In 2012, Chip Brock said to me, now that we know that the Higgs boson is there, we finally have to take the questions about it seriously. Our knowledge of the Higgs boson also dramatically sharpens our questions about electroweak symmetry breaking. Only those models survive in which the Higgs field can be a weakly-coupled entity, a scalar field or at least an effective scalar field produced at much higher energies.

I think it is quite under-appreciated how much the LHC has enriched our knowledge of QCD. At high energies, even perturbative QCD is complex, giving rise to jets whose internal structure is close to a fractal. Events with 5 or even 10 jets in the final state are not uncommon at the LHC. And QCD explains these layers of structure with a high degree of precision. This is an amazing success bringing together theory, simulation, and experiment.

And, for physics beyond the Standard Model, the LHC has been a source of creative destruction. Whatever is the truth of the arguments that I have given early — I still find them compelling — the models that we created are too simple. Essentially all of the parameter sets considered in the 2000’s, including possibilities thought to be difficult for observation at the LHC, have been ruled out. It is still possible to tweak the parameters and push the particle masses to higher energy. But it seems that some idea is still missing.

What can we learn from a Higgs factory?

I believe that the Higgs factory provides our next big opportunity to access physics beyond the Standard Model. This requires some explanation, especially about the competition with LHC and HL-LHC. So, let’s discuss the various aspects of this statement.

The LHC still has some way to go to reach its final results. The HL-LHC will give us twenty times more data than we have now and, although it won’t increase the pp center of mass energy significantly, it will substantially extend the reach of searches for new physics. This is especially true in the electroweak sector, where the cross sections at hadron colliders are relatively small. We have to keep doing searches and hope that some interesting signal will show up.

As I said before, 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 a more composite- Higgs, strong-interaction type of model of electroweak symmetry breaking. The LHC has set limits on colored top partners at about 1.3 TeV. The HL- LHC will move the limits up, maybe as far as 2 TeV, but alternatively we will discover something along the way. On the other hand, we are now seeing the limits of the LHC's reach. New particle discovery is not such an open field as when the LHC began data-taking in 2010.

Higgs factories offer a different opportunity to discover physics beyond the Standard Model, one that I would say has not been opened yet. This is the exploration of the couplings of the Higgs boson itself. The Standard Model predicts these couplings with very high precision based on the measured inputs, including the Higgs boson mass. The LHC now has tested these predictions at the 10-20% level and finds agreement with the Standard Model. But this is not a high enough level of precision to be sensitive to models of new physics, which typically produce deviations from the Standard Model below the 10% level. HL-LHC will improve the current precision tests, but an e⁺e^- Higgs factory will take us to a new qualitative level, measuring the full spectrum of Higgs boson couplings at the 1% level or below.

At this level of precision, it should be possible to discover a pattern of deviations in which the different Higgs couplings respond differently. Each BSM model has its own pattern. If we can see the pattern that nature chooses, that would point to a particular direction among the many possibilities for extending the Standard Model.

I think that studying the Higgs boson at an e⁺e^- collider is essential to this study. Sometimes people compare the projected capabilities of e⁺e^- colliders to estimates of how well the HL-LHC experiments could possibly do. But it is important to think beyond simply shrinking the error bars. If the Higgs boson couplings deviate from the Standard Model, we will need to prove that the deviations are there, to convince a skeptical audience both within our field and in the broader community. At hadron colliders, it is a feat even to recognize Higgs boson events and to extract them from very similar backgrounds. If we also demand precision, we must consider the delicate issues of how events are modeled and classified. At an e⁺e^- collider, this first step is just much simpler; to a first approximation, Higgs events stand out. Then we can devote our cleverness to controlling systematic uncertainties much more powerfully and eliminating them as a source of confusion.

I think that this study is an opportunity for the next generation of collider physicists. Going beyond direct particle searches, an e⁺e^- Higgs factory will give us the chance to prove that the Standard Model is incorrect and to uncover clues to the direction in which it should be modified. I have already explained above that those new forces and new particles are out there, This is the best chance that we have now to prove their reality.

Doesn’t this mean that we also need an accelerator beyond a Higgs factory?

Yes. Before the LHC, we could hope that we would find the explanation for electroweak symmetry breaking, dark matter, and the other issues of the Standard Model at the LHC. Steven Weinberg famously dreamt of a “final theory” that a machine sensitive to the TeV scale would discover. Now, whether the HL-LHC discovers new particle, the e⁺e^- Higgs factory discovers new interactions of the Higgs boson or the top quark, or flavor experiments discover anomalies that require BSM explanations, we expect to find only clues to a structure of new physics whose main components lie at higher energies. Eventually, we need to go there. It is a big challenge to design an accelerator that can go to energies much higher than the LHC.

Today, we do not have any acceptable technology. For a proton-proton collider, we need magnets that are many times stronger than those used in the LHC. There are designs for such magnets, but the prices are currently unaffordable. For a muon collider, we need to learn how to cool muons, that is, to decrease the phase space of a muon beam by a factor of a million. Not much progress has been made over the past 20 years, but there is a new initiative now. For an electron collider, although we know how to accelerate electrons at high gradients over a few meters, it is much more challenging to maintain these beams for acceleration over kilometers. 

These are all fascinating research questions. I hope that more young particle physicists will get involved in the accelerator research needed to solve these problems. It may be their best way to contribute to the future of particle physics.

In your opinion how the community should think in choosing one of the proposed colliders?

I suppose that you are asking here about the next-generation machine, the e⁺e^- Higgs factory. I think that there is general agreement in the world now that the physics goals of the Higgs factory are important. But we are not making progress toward actually getting a Higgs factory built. These machines are expensive at the level where global cooperation and cost- sharing is needed. But the current initiatives are all regional. CERN is planning the FCC, which begins with a circular Higgs factory. Though FCC is a program of 60 years or more for CERN, the Higgs factory is not coming  for a long time due to CERN’s ongoing commitments to the LHC.

In Japan, there are discussions of hosting a linear Higgs factory, the International Linear Collider (ILC), but the progress toward funding it has been slow. China is also proposing a large circular machine, but again this has missed several 5-year plan deadlines. In the 2000’s, the US considered hosting the ILC at Fermilab, but that idea failed due to the cost. Now that there is increasing interest in the physics goals, we can try again to have a linear collider based in the US. We are also proposing a new linear collider technology, called “C-cubed” as a possibly less expensive option.

These solutions are based regionally, but actually any technology could be built in any region. To make this happen, though, we need a plan based on global cooperation. If we agree that physics is important, the resources are there to construct a Higgs factory that takes data during the careers of the young people now working at the LHC. I hope that the current P5 process in the US can make this a priority and put us on a path to realizing this opportunity.

Which do you consider to be the starting point of QFT and its future?

When the Standard Model was developed, it was made out of quantum field theory ingredients. Yang and Mills wrote down the Yang-Mills theory in 1954. They proposed it as a theory of vector fields coupled to strong interaction isospin. That idea turned out to be completely wrong, but the equations were so compelling that it seemed that they must have some fundamental application. It turned out that Yang-Mills theory could be applied in two different ways to build the Standard Model. In QCD, these equations give a model of composite hadrons based on quark confinement. In the electroweak theory, the same equations give a model of massive vector bosons based on spontaneous symmetry breaking. From the time of the Yang and Mills paper, it took almost 20 years, and much insight gained from both theory and experiment, to reach the final product.

To build models of physics beyond the Standard Model, we probably need more ingredients from quantum field theory, including ingredients that we do not know about today. Probably the biggest idea in quantum field theory since the discoveries of Yang-Mills theory and the renormalization group is supersymmetry. We have learned a great deal about supersymmetric models — including the dynamics of strongly coupled supersymmetric theories — but we still do not know how to turn that knowledge into a compelling theory of nature.

We also have much to learn about strongly coupled quantum field theories. Our current ideas about strong coupling seem too simple, and they have not yet led to successful models of composite Higgs bosons or composite top quarks. I hope that there are new ideas that are still undiscovered.

More generally, there are many even bigger questions about quantum field theory. Presently there is a lot of interest in the calculation of scattering amplitudes. In QCD, the results of complex perturbative calculations are often much simpler than what one might have expected. This suggests that there is an alternative formulation of quantum field theory in which the route to these answers is more transparent. This new formulation would most likely give up manifest locality in space-time. Nima Arkani-Hamed is one of the very vocal proponents of this point of view. In his lectures, he often raises the point that considerations of quantum gravity tell us that we must replace local space-time with some alternative new underlying principle. We need some really good ideas to explore this question! There are many hints now to what is needed. I hope that someone will succeed in bringing them together.

What is the link between particle physics & cosmology?

Since the 1990s we have learned a huge amount about the nature of the universe. We now know that the dominant form of mass in the universe is dark matter, a form of matter not accounted for by the Standard Model. We know that most of the energy content in the universe is in the energy of the vacuum — “dark energy” — and this adds to the mystery of the small size of the cosmological constant. We have increasingly sharp evidence that the structures we see in the universe, such as galaxies and clusters, evolved from a primordial set of perturbations that were approximately Gaussian and scale-invariant. These are the fluctuations of an almost free scalar field, as postulated in the theory of inflation. An origin of the universe in inflation requires that the baryon-antibaryon asymmetry be generated by particle physics in the early universe. But this requires CP violation. The improved understanding of the CP violation in particle physics that we have after the B factory experiments makes it clear that a different origin is needed for cosmology. Each improvement in our knowledge seems to bring a new problem that requires new, undiscovered, laws of physics.

All of these problems seem to test our ignorance of scalar fields. Models proposed for all four of these problems involve new scalar fields of one kind or another. And quantum field theory seems to give us no guidance about the couplings of scalar fields. It seems that we can just put in any numbers that we wish. In fact, that freedom is used to propose solutions to the problems of cosmology. Maybe there is some new aspect of quantum field theory that we need to learn here. Maybe understanding the Higgs field — the one scalar field that we can study experimentally in detail — will help us discover it.

Michael Peskin in the librafy of the Institute for Particle Physics/Instituto de Física Corpuscular (IFIC, CSIC-UV) in Valencia Spain holding his classic textbook on Quantum Field Theory. The photo was taken during his visit in September 2016 to deliver a lecture on the Mysteries of the Higgs boson. 

How did you decide to write your two well-known books?

Particle physics is a very beautiful subject, but it is also a subject built on many difficult concepts. Students struggle to get a toehold. Even senior experimenters freely confess their lack of a deep understanding (and senior theorists feel this also, though they would never admit it). So we ought to put effort into making the basic concepts of quantum field theory and particle physics as clear as possible. By this I mean not only that we should write some simple equations that students can memorize and work from, but also that we should make clearer why these equations are as they are and what surprising features of the universe they give rise to.

I think that I have been lucky to be able to make some contributions here. It was clear in the 1980’s that a new standard textbook of quantum field theory was needed. The classic 1960’s text by Bjorken and Drell had served the field well, but so much had been learned since it was written that someone needed to start over and explain the theory from the beginning in the light of that knowledge. Many people attempted this, but I think that the textbook that Dan Schroeder and I wrote has proven to be the most successful. It is a huge amount of work to write a textbook but, if it succeeds, it is very rewarding to hear from students who have learned from it. The book is now more than 25 years old and is still going strong — though maybe this is because the next revolution in quantum field theory has not happened yet.

More recently, I have written a textbook of particle physics, pulling together approaches and materials from many years of university and summer school lectures. I titled the book “Concepts”, and I think that this emphasis is important. Students should know more about the Standard Model than they can find on a T-shirt. They need to know how it grew from its origins and what deep questions it does or does not address in order to frame questions about its future. It remains to be seen how well this book will succeed. This will depend on how well it meets the needs of students. It also depends on whether professors treat particle physics as a closed subject or as one in which we must prepare for new discoveries.