# Interview with Roberto Peccei

Roberto Peccei, is a world renowned theoretical physicist working in the interface between astroparticle physics and cosmology. He is probably best known for his work on strong CP violation. Together with Helen Quinn, Peccei suggested a mechanism to address the strong CP problem, a major blemish of the Standard Model of particle physics- namely its failure to explain why the fundamental laws of physics in the strong interactions look the same if you run time backwards.

Peccei and Quinn received in 2013 the prestigious Sakurai Prize for Theoretical physics “for the proposal of the elegant mechanism to resolve the famous problem of strong-CP violation which, in turn, led to the invention of axions, a subject of intense experimental and theoretical investigation for more than three decades."

Born in Italy, in 1942 Peccei decided very early that he wanted to become a physicist. “I was always fascinated by the big scientific questions about the Universe. However, my desire to study physics came from a science prize that I won in high school. The prize included a book on atomic energy that I found fascinating. That’s when I decided that I wanted to become a physicist.”

Following his father advice, he applied to the MIT’s department of Physics and got accepted. He obtained his B.S. in 1962 and completed his PhD in 1969 writing a thesis on “The chiral dynamic method and its applications in high energy physics”. At MIT is where he first met Steven Weinberg, who later with Frank Wilczek noted that the Peccei-Quinn mechanism implied the existence of axions.

“In the mid 60-s things were rather complicated in particle physics and it was not always easy to choose the right path to follow. One of the hottest topics in fundamental physics then was the resurgence of field theory. I got interested in effective Lagrangians and I worked during my PhD on effective Lagrangians for threshold π-N scattering. “

Following his PhD, Peccei became a post-doctoral fellow at the University of Washington in Seattle and after that was offered a position as Assistant Professor at Stanford. “This is where I first met Helen Quinn, who was a visiting Professor from Harvard University. We both had similar interests related to the existence of instantons in the strong interactions and soon we started working together. One of the first things that we tried to understand was how instantons lead to fermion number violation.

During that time, Steven Weinberg was also visiting Stanford and liked to discuss physics during lunchtime. Quinn and Peccei often had lunch with him and enjoyed the chance to discuss their research with him. “Weinberg brought up the so-called θ problem in strong interactions. Why no CP violation was measured by the experiments? Could this θ be a parameter that goes to zero values”. Peccei and Quinn were stimulated by Weinberg to think about this problem, partly because he continuously was asking them whether they had a solution to this conundrum!

“At some point, we realized that we could naturally arrive at a zero value for theta by introducing a new symmetry”, that is today known as the Peccei-Quinn symmetry. “You can use this symmetry to turn theta from a constant parameter to a dynamical one and thus explain the lack of any CP violation in strong interactions”.

The strong CP problem can be solved essentially three ways: assume some unconventional dynamics or a spontaneously broken CP symmetry or a third approach is to introduce an additional chiral symmetry. This is the path pursued by Peccei and Quinn. Peccei notes that the “Peccei-Quinn” symmetry and is the only viable solution that remains. “Of course θ could have an infinitesimal small value of O(10−10) due to some anthropic reason but I doubt it as a Universe where CP is strongly violated seems as viable as one where it is not”.

Introducing an additional chiral symmetry to the Standard Model is a very natural solution for the strong CP problem since this chiral symmetry, effectively, rotates the θ parameter away to zero. This symmetry is spontaneously broken and introducing it to the theory essentially replaces the static CP-violating θ angle with a dynamical CP-conserving field. As a spontaneously broken symmetry, there is always an associated pseudo-Goldstone boson and its field is the so-called axion field. Peccei says: “We were very excited to having solved the strong CP problem that we didn’t delve in the consequences of this symmetry. Weinberg and Wilczek, independently, understood that the Peccei-Quinn symmetry implies the existence of a new particle. Both of them were generous to give us credit for the PQ symmetry idea!”

The axion is the resulting Nambu–Goldstone boson and its field, a, can be redefined to absorb the parameter θ. While initially massless, non-perturbative effects, which make QCD θ dependent, result in a potential for the axion. This potential causes the axion to acquire a mass and relax to the CP conserving minimum, solving the strong CP problem. Furthermore, as there are no degrees of freedom available for the axion in the Standard Model, new fields must be added to realize the PQ solution.

Originally, the axion was thought to be very light, with a mass inversely proportional to the the EW scale. “The idea was that the scale breaking of the PQ symmetry is the same as the scale of weak interactions. That meant that axions should have a mass in the MeV or sub-MeV range and originally many people thought that axions should be closely related to the Higgs particle. In addition, the axion field could be very important for the development of the early Universe and axions should have been abundantly produced during the earliest moments of the Big Bang”

“The original model was long ago ruled out by experiment but one can assume that the PQ symmetry breaks at much larger energy scales which consequently means that axions should be much lighter”. Indeed, more recently, attention turned to much smaller values of the axion mass (and in consequence feebler couplings), which are not excluded by experiments. Axions of this sort arise very naturally in models that unify the interactions of the Standard Model and can be very good candidates for dark matter due to the fact that they still carry mass while interacting very weakly with the rest of our Universe. “This part revived a lot of interest for research in this field and I find exciting the fact that currently there are so many axion experiments going on”.

Following his work on formulating the PQ symmetry, Peccei was involved in many different models that incorporated this symmetry. “What is clear to me is that the best solution of the CP problem involves introducing an extra chiral symmetry. Such a solution, necessarily, predicts the existence of a concomitant axion. However, what remains an open question is the origin of this symmetry and its connection to the early Universe. I believe that there is a lot of room for thinking and trying to understand what might give rise to this symmetry”.

Today, Fermi scale axions have been ruled out, but invisible axions models like the so-called, KSVZ and DFSZ models, are still viable. Axion oscillations toward its minimun could account for the dark matter in the Universe. “This is an exciting possibility. No totally compelling invisible axion models exist, and there are no strong arguments to believe that the symmetry breaking scale takes precisely the value needed for axions to be the dark matter in the Universe. Nevertheless, it is encouraging that experimentalists are actively searching for signals of invisible axions.”

As for the future, Peccei thinks that we should continue exploring the energy frontier and intensity frontiers aiming for higher energies up to 100 TeV. “Of course it is more speculative to argue for a higher-energy machine now, compared to the LHC for which the Higgs was a very strong motivation. However, looking at the history of the field this is the way forward. Going to higher energies or higher precision could reveal some discrepancies that can provide hints for the new physics to answer the open questions that leaves the Standard Model incomplete”. He continues “understanding what stabilizes the Higgs mass to such a low scale, preventing it from going up to the Planck scale, or exploring the nature of dark matter are two of the biggest challenges for the next machine. There are many guesses but we need more experimental evidence”.

Peccei believes that to understand the universe we need to explore different frontiers, both in astronomy and in particle physics. “You really have to look at a broader gamut of disciplines. I think four centuries ago, if you wanted to understand the universe, you did need to invent the telescope that looked at the universe. Now, you need many more tool. You are trying to bring all the people that have tools that will help us understand the universe together. I think that that is the mission, perhaps a bit broad but is the right mission.”

Finally, Peccei notes the importance of collaboration for advancing in fundamental physics. “I see young people working very much in the same style as we used it. You look at a problem, you attack it in various ways, and together with your colleagues, you postulate and comment on new ideas. That kind of interaction still exists and is vital for our field”. As to his advice to young physicists: “Be persistent, patient and follow the physics. This is what I always did. Although I worked on the strong CP problem while at Stanford I didn’t get a tenure there. This is when I moved to Europe, where I spent a few great years in the Max Planck institute and later at DESY before returning to the U.S. It was a period during which I had to work hard to prove myself and I am happy that I did!”.

** Top image credits: Romanian Association for the Club of Rome**.