CERN Accelerating science

Interview with Helen Quinn

by Panos Charitos

Helen Quinn, a professor emerita at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University, received the 2018 Benjamin Franklin Medal in Physics last April for her accomplishments in the field of particle physics.  

As a student, Quinn says, she grew up in an environment that encouraged curiosity about how things work and about identifying connections between different parts of our world. She recalls her math teacher saying “Helen, you could be a mathematician because are so lazy; you will never solve a problem the hard way, but always have to find a better way”. “I am not sure if she wanted to praise me but somehow I felt that she supported my tendency to want to understand mathematics at a deeper level, which was critical to my later career.”

Quinn started her undergraduate studies in the University of Melbourne. With some financial support from the Australian Weather Bureau, she registered to study meteorology. “If my family hadn’t decided to move to the US, I would have become a meteorologist! In 1962 we all moved to Palo Alto and I was accepted at Stanford. I quickly started looking in different courses of study that would allow me to get my degree and decided to study physics. This was partly thanks to Jerry Pine, now a Professor in Caltech. He gave me the flexibility to attend different classes and decide where I wanted to place myself in a way that I could complete the degree.”

Photo from Hellen Quinn's passport when she arrived to the U.S.A. 

Quinn’s interest in particle physics began at Stanford in 1963, after completing her undergraduate education in physics. “My advisor encouraged me to move on and apply for a graduate programme but said ’Many graduate schools are reluctant to accept women because they get married and they do not finish. But I don’t think we need to worry about that with you.’ This left me thinking ‘Is he telling me I’m never going to get married or trying to encourage me?’”. Princeton wrote back saying they did not normally accept women. Stanford, however, did accept her as a graduate student.

“I was lucky to be there at the time when the Stanford Linear Accelerator Center was being built.” SLAC was the world’s longest linear accelerator and promised unprecedented explorations of the subatomic world. “Really, the beginning of my interest was the fact that particle physics was bubbling at that time at Stanford, and that’s where I got hooked on it.” Quinn thinks that an important element in any scientist’s career is the opportunity to meet inspiring colleagues and interact with teams working at the forefront of research. Enthusiasm for a problem seems to be contagious, she says.

After graduating from Stanford, Quinn pursued research at the German research center DESY and at Harvard University. During her graduate studies in Stanford, she met and married Dan Quinn, then a student of experimental particle physics. They applied together for postdoctoral positions at DESY and were both accepted. Two years later, her husband was offered an Assistant Professor position at Tufts University. “I thought, well if there is a city in the country where I could also find an academic position, it is Boston! But I did not find a job that year. I thought about becoming a high-school teacher and registered for some education courses in Tufts, which included practice teaching in a local high school. Perhaps this is when I began my preparation for my later work in science education!” Eventually, Quinn decided to continue with her research in particle physics, first as a visitor and later with a position at Harvard. She collaborated with theorist Steven Weinberg and Howard Georgi to publish a paper on “Hierarchy of Interactions in Unified Gauge Theories” that introduced a general formalism for understanding how the strong, weak and electromagnetic fundamental interactions merge at very high-energy scales. This paper is one of those that led to her Franklin award and other recognition.

Quinn made her next move by taking a leave of absence from Harvard. “In 1976, we moved back to California because my husband was changing his career from physics to decision analysis and that’s where he found a job. I used my Sloan fellowship to visit Stanford for a year, then began looking for a job around Stanford. SLAC could offer me only a temporary research position, so from an Associate Professor at Harvard I found myself again as a post-doc at SLAC” until the following year, when Quinn became a member of the SLAC permanent scientific staff.

During the year at Stanford, she worked with Roberto Peccei on another highly recognized contribution, now called Peccei-Quinn Symmetry. “The question behind our work was: how come the CP violation in weak interactions doesn’t infect strong interactions? What is the mechanism that protects strong interactions from this violation?” CP is a symmetry between the laws of physics for matter and those for antimatter. It is known to be violated by weak interactions, but experimental upper limits on the electric dipole moment of the neutron put very tight limits on any strong interaction CP violation. In the strong interaction theory, such effects can enter; the parameter that defines their size is called theta.

“We started exploring this problem by thinking about cosmology! If quarks had zero masses — which we assume is the case for quarks in the early Universe— then one could tune away any theta parameter by redefinitions of quark field phases. Roberto and I prepared the first draft of a paper based on this idea, but it was completely wrong! We talked to Steven Weinberg, who was then visiting Stanford and he made a comment that helped me recognize that, by tuning away the theta parameter, you change the phase of various Yukawa couplings.  Although the Yukawa couplings are also invisible in the high energy situation of the early Universe, they are the places where CP violation is hiding until quarks get their masses. So we did not tell Steve our wrong idea and went off to think some more.

We now understood that the solution was linked to the Yukawa couplings. “I saw that we needed to add a symmetry to arrange them so that the effective value of theta after the quarks get their masses would be zero, no matter what it was in the high energy situation. The trick is that the symmetry links the phase of the quark mass matrix to the theta parameter, so that the combination, which is quantity that matters for CP violation, is naturally zero once the Universe cools enough that the quark masses appear. To allow the symmetry, the theory added additional Higgs-type scalar particles to the Standard Model.”

Weinberg and, separately, Wilzeck immediately saw that the Peccei-Quinn theory suggests the existence of a very light but yet-to-be-found particle, called the axion. Quinn remembers that “Roberto and I were so excited to have solved the original problem that we decided to quickly move forward to publish our idea with the simplest possible example model. This was only a toy model. We did not think it was realistic so we did not further explore its phenomenology. We were just using it to illustrate how the symmetry worked to solve the CP problem (laughs). “So we did not notice that the model also illustrated another general principle, according to which there should be a light scalar particle associated with the symmetry. This is the axion! In fact, Weinberg, before publishing his paper, called me and asked me,  ‘Did you notice…’, and I paused for ten second before replying, ‘Of course this is a consequence of our theory, but no, we had not noticed it’”.

The first Peccei-Quinn model was soon ruled out by experimental tests. Versions that have more additional Higgs fields add more parameters and give the flexibility to evade those experimental limits. Following a suggestion from Pierre Sikivie, experiments are now searching for axions as a possible explanation of dark matter. Observing them, or any other additional scalar particles would be very interesting. Today, the term axion is used for any very light scalar particle with relatively little interaction with anything else, though it does interact with photons. Other axion searches probe for these particles in parameter regions that would not work for dark matter.

Quinn notes that even if searches for DM axions exclude that possibility, this won’t rule out the Peccei-Quinn theory. “It simply rules out that axions are the dark matter in our Universe. You know, it is very hard to exclude a theory that predicts something that is very hard to detect”.

Quinn reminisces at the changed state of particle theory, saying “I started before we had any of the current theory and now is a very different time. We had the Fermi four-fermion theory for weak interactions, which we knew didn’t work beyond leading order, but we were not too worried because that was good enough to make estimates. There really was no calculable theory for strong interactions. Those were considered to be mediated by rho and pi mesons”. She adds: “We also had Current Algebra and Regge theory that were useful, and still apply, but are incomplete. At the same time, there was Jeff Chew’s bootstrap model claiming that no particle is elementary. Particle physics seemed very confused.”

The early 1970s brought big changes when the pieces of the Standard Model were pulled together. At that time she was a student-teacher at a high school and Joel Primack, a junior fellow at Harvard and a good friend of Quinn invited her to visit theorists at Harvard University, and the conversation led her to return to research “At that moment, a piece of work had just appeared that was really fundamental to the development of the Standard Model. In 1971 Gerard ’t Hooft and Martinus Veltman published a new method for calculating in gauge theories, which underlie the Standard Model. Their paper allowed many physicists to revisit Weinberg’s paper “A Model of Leptons” that is a key piece of the Standard Model today. Although it was published in 1967, it had very few citations before 1971 because no one knew how to make calculations beyond the leading order.”

Together with Tom Appelquist, Primack and Quinn set to work to apply the ‘t Hooft and Veltman approach. “Before the Standard Model, there was a problem with weak interaction theory. You could do the first-order calculation, but the next order (the one-loop calculation) was infinite. So the old theory was not well-defined nor stable. We did the first finite one-loop calculation of weak interactions (muon decay) using the new theory. In that paper we also mentioned that the strong theory needed to be a vector-like theory so as not to disturb the good results. I remember how pleased Steve Weinberg was with our result.”

Over the next few years, developments extended Weinberg’s lepton model to quarks and the charm quark was predicted and discovered. Parallel developments built the QCD theory of the strong interactions of quarks, also a gauge theory. By 1976, John Iliopoulos in a talk in the Lepton-Proton conference could put all the pieces of the Standard Model together. “It was a great period and a stimulating intellectual environment. During my years at Harvard, Steven Weinberg was there, as well as Appelquist, Georgi, Glashow, DeRujula and others. David Politzer was a graduate student.”

However, Quinn is less optimistic about the modern picture of particle physics which is very different compared to the 1960s. “Currently we have an incredibly good theory up to a certain scale and indirect evidence that there must be something more, possibly at very much higher scales. It is extremely hard to probe those scales. We can speculate and build very nice theories but we have little evidence to test them. It may be that we will not get access to an energy scale that can give us evidence to distinguish among all the speculations and reveal more of nature’s reality. We have gone from having lots of unexplained data but little successful theory in the 1960s to having a very successful theory and little data that points the way to go beyond it today.”

Following her retirement in 2010, Quinn has focussed on science education. She chaired the National Research Council committee that produced “A Framework for K-12 Science Education,” an influential report that served as the basis for the 2013 Next Generation Science Standards. Essentially, in the US, standards are the basis on which the States build assessments, and thus they’re a set of guideposts for teachers and curriculum developers about what to teach. “Over 30 states in the US have now adopted standards based on the Framework and it has also had major influence internationally. However, by their nature, standards are bits and pieces of knowledge. It really helps to have a broader picture, such as that given by the Framework to express the vision for science education; the guideposts alone are not enough.”

Hellen Quinn and the Franklin Award sponsor Larry Gladney (also a physicist) at the award ceremony. (Credits: Dan Burke Photography)

Science education remains her primary focus. Quinn finds the work rewarding and significant, albeit in a more pragmatic way than developing new particle physics theories, as it can impact many more people. “We are aiming not only to educate physicists and other scientists for tomorrow but also to make science and scientific thinking available to everyone. Everyone needs to make decisions for themselves or their community and to be able to look for evidence to support or refute the information they are given as they are confronted with complex questions. Science education, done right, gives us the knowledge and capacity to think through such decisions in a different way.”

Quinn adds: “I want every citizen to have that capability because that can affect the future in ways that are constructive and positive. I want citizens who can look at a problem in their community and think like a scientist about the part of the problem that is science. I also want high school and college graduates equipped with capabilities that employers want, whether they come from well-educated families or not. Science education has an important part to play in making this a reality.”