Underneath It All: Mary K Gaillard’s Adventures in the World Subatomic

The sub-atomic world as we learn about it in high school is a seemingly settled and staid affair - you have protons and neutrons in the nucleus, and much smaller electrons orbiting about them. Electrons might hop to a higher level when they get hit with the right photon, and some neutrons and protons might leave if the atom is too big, or change into each other to preserve stability, but by and large the tableau is one of serene order, with everything in its right and proper place.

Students still get taught that today, but it stopped being completely true a century ago, and was thoroughly exploded a half century ago in the great particle revolutions of the 1970s. Not only is our universe populated by a panoply of weird particles that live for quadrillionths of a second, but even our stately friends, the protons and neutrons, are, beneath their surface, a hotbed of activity, a bustling marketplace governed by the exchange of particles that shouldn’t exist, but that the existence of matter itself is entirely dependent upon. The people responsible for this breath-taking and at times baffling view of the universe were of a generation now passing beyond the veil - Murray Gell-Mann passed away in 2019, Steven Weinberg in 2021, and Tsung-Dao Lee and Peter Higgs both in 2024 - but none of those hit quite so hard as the recent death of Mary K Gaillard on May 23, 2025. 

(By the by, before we go on, I’m happy to report, as of the time of this writing, CN Yang, TD Lee’s partner on theorizing the non-conservation of parity in weak interactions, is actually still with us at the age of 102!)

Born Mary Katharine Ralph in 1939, Gaillard was of a generation of women who came of age in the 1950s, when Eisenhower era social norms dictated that the goal of a woman was marriage to a stable provider, and a lifetime of managing the domestic economy. Though she had the benefit of two educator parents who, both by their actions and their words, demonstrated to her their fundamental belief that women and men had the same academic abilities, Gaillard’s chosen field, that of particle physics, was one rife with sexist assumptions that would continue to have their head firmly in the 1950s well into the 1990s, and navigating those assumptions was to be a constant in Gaillard’s career.

Gaillard at Columbia

In high school, Gaillard was something of an all-around academic force, interested in art, drama and music as much as the math and science she excelled at. When it came to the sciences, her school only required two years of study, to be chosen from general science, biology, chemistry, or physics, but the requirement of animal dissection in biology decided her in favor of the latter two, and she found that physics, with its strong core of mathematics, particularly appealed to her.

She wanted to pursue physics in college, but the family finances severely limited her options on where she could study, so instead of attending a premiere physics institution, she went to the school that offered the best scholarship, Hollins College, in Roanoke, Virginia. It was here, at Hollins, that she found herself drawn into the orbit of Dorothy Montgomery, who had joined the physics faculty in 1946 and was known for having done research with J. Robert Oppenheimer. Montgomery was one of two physics professors at Hollins, and took Gaillard under her wing, using her connections to secure the hopeful physicist a place in the lab of Louis Leprince-Ringuet (1901-2000) at l’École Polytechnique in Paris during her year of Study Abroad, and advising her to apply for an internship at Brookhaven National Laboratory during her junior and senior summers. 

Brookhaven was a center of particle physics research at the time, thanks to the Cosmotron particle accelerator that went online there in 1952. The Cosmotron was at the end of its run as the superstar of Brookhaven by the time Gaillard arrived in the late 1950s (it would soon be superseded by the legendary Alternating Gradient Synchrotron, discoverer of the muon neutrino and charm quark), and indeed during her first summer there it was down for repairs the entire time, but during her second summer she had an opportunity to do some cutting edge physics work, collaborating with Columbia researchers probing an expected new high energy behavior. She also met her future husband, physics post-doc Jean-Marc Gaillard, during this Summer, setting in motion a series of events that would bring her to the heart of particle physics research but also keep her at arm’s length from the support that her training and success merited.

That all lay in the future, however, when she graduated from Hollins in 1960 and proceeded to pursue her Master’s degree at Columbia after taking a few brush up courses recommended by Montgomery at Case on thermodynamics and field theory. It was at Columbia that she began running into the phenomenon that she would encounter repeatedly at the beginning of her career - as a woman her professors and colleagues expected her to gravitate towards experimental rather than theoretical physics. The prejudices of the era declared that, while women might be good with the practicalities and finesse of creating and running equipment to test theory, they simply were not suited to the large-scale abstract thinking that lay at the heart of theoretical physics, and so Gaillard was repeatedly urged in the direction of experiment even though her interest and talents clearly lay in the direction of theory.

In 1961, she married Jean-Marc Gaillard, and in the years that followed her fate was largely determined by the course of his career, a situation common with many wives in scientific pairs in that era. She soon followed him to France where, in spite of having a graduate degree from Columbia, and experience working at one of the world’s leading particle physics centers, she struggled to find anyone willing to take her on as a graduate student. She was offered positions insultingly below her talents as a scanner, rejected from one work group after she got pregnant and the organization said they couldn’t honor her contract because of the radiation danger to the baby, and generally told that, to advance in France, she had to have studied at the École Polytechnique or École Normale, neither of which were open to women.

These frustrations were interrupted by a snap return to the United States, when Jean-Marc wanted to be at Columbia for the Two Neutrino experiment that he had contributed to, which ran from September of 1961 to June of 1962 and ultimately succeeded in finding a second type of neutrino, what we know today as the muon neutrino. This was a major result which Gaillard had also contributed to, but she mainly spent this time taking courses to ensure that she would pass her exams back in France. These included taking a nuclear physics course with the most revered woman physicist of the age, Chien-Shiung Wu (1912-1997), as well as guided readings in statistical mechanics. 

Gaillard with Nicola Cabibbo and Tini Veltman

Soon after returning to France, Gaillard moved again, as Jean-Marc received a staff position at CERN, and so in 1964 the couple was off to Geneva, where Gaillard would spend the next 17 years producing a series of scientific triumphs while consistently being denied an official position at CERN commensurate with her research. A great deal of her early work revolved around kaons, otherwise known as K-mesons. Unlike protons and neutrons, which are made of three quarks, K-mesons are made of only two. Where things get weird, though, is what kind of quarks they are made out of. Protons and neutrons are made of up and down quarks, which are the lightest, most stable of the six quarks that exist in nature, but kaons are made of a strange quark or antiquark combined with an up or down quark or antiquark. So, not only do they have strange quarks in them, which are some twenty times more massive than the down quarks whose charge they share, making them correspondingly less stable, but they also combine matter and antimatter within the same particle.

These are weird dudes. While protons are essentially eternal, and neutrons have lifespans measuring some fourteen minutes when out roaming free, the longest lived kaons live for something on the order of .000000012 seconds, and the shortest lived ones die a thousand times faster than that. Where Gaillard entered the picture was in the great debate of 1964 about how different types of kaons decay. There is an idea in physics called CP parity, which says that if you replace every particle in the universe with its antiparticle (C parity), and look at every process in the mirror (P parity), nothing will change. In 1964, Gaillard suggested a method to catch kaon decay in the act of breaking CP parity. The paper put her on the board, and was considered important enough that other scientists tried to plagiarize it, though it didn’t bring a change in her status at CERN.

That year, long kaons were caught decaying, .3% of the time, into two pions instead of their usual three, a result which broke CP parity, and opened the gates for a flood of new speculations in physics, including possible explanations about how our universe ended up with more matter in it than antimatter. 

Gaillard was at the heart of the new physics being developed, and was at a very young age contributing substantively to it, all while juggling the home and childcare needs that came with being married with three children. Somehow, with the help of a series of au pairs and babysitters, she managed the chaos, and continued her research into kaons, proposing a mechanism for how kaons decay into a pion, a muon or electron, and a neutrino, that agreed with experimental results, and which made predictions about the relative masses of the up and strange quarks that represented some of the early steps in the development of quantum chromodynamics.

By the late 1960s and early 1970s, Gaillard was recognized as a world expert on the phenomenon of weak interactions (those which allow quarks to change their identity, or “flavor”), and divided her time between work at CERN in Geneva, and at Fermilab near Chicago. The new quark on the menu at the time was the charm quark. Just as the strange quark is a significantly heavier version of the down quark, the charm quark was hypothesized as a heavier version of the up quark which, if it existed, would explain why some weak processes that altered a particle’s strangeness were allowed while others weren’t, and would allow for the unification of the weak and electromagnetic forces into one unified, electroweak, force. Gaillard, working together with Ben Lee, delved into her expertise with different kaon decays to produce a calculation in 1974 that predicted a mass of around 1.5 GeV/c^2 for the charm quark, some three times smaller than the value predicted contemporaneously by Ernest Ma. 

The most recent measurement of the charm quark’s mass places it at 1.27 GeV/c^2.

During these years, she also worked on the problem of why weak decays that transformed hadrons containing strange quarks into hadrons containing normal quarks changed the isospin of the respective particles by either one half or, far more rarely, three halves. Though the end result did not turn out to be the accepted explanation, the techniques that she developed along the way became important ones for the analysis of low energy weak processes. 

Gaillard and John Ellis

By the mid 1970s, then, Gaillard was something of a rock star in the world of particle physics, with important work in kaon decay, charm mass, the ΔI=1/2 problem, the prediction of “three-jet” events, the computation of the bottom quark’s mass, and emerging work on the production and decay of the as-yet undiscovered Higgs particle. Though she was promoted within the CNRS (National Centre for Scientific Research), CERN itself continued to deny her a regular paid staff position, resulting in a variety of absurdities on the international conference scene where she would be representing CERN results without having CERN meaningfully contribute to the cost of her travel, or to the funding of her position. She was only given a partially funded position as a visitor at CERN in 1979, and in 1981, CERN declined definitively to offer her a permanent position. 

This was the last indignity, and Gaillard promptly took advantage of the numerous offers she had of employment in the United States to leave France at last, divorcing her husband two years later. Once in the States, she ultimately decided to plant her flag at UC Berkeley, becoming the first woman physics professor there, while also serving as a senior staff member at the Lawrence Berkeley National Laboratory. These were the years when Supersymmetry seemed the best hope for solving the problems that lingered after the Standard Model was largely finalized in the mid 1970s, and Gaillard threw her theoretical gifts into the plumbing of different models Supersymmetric models, particularly N=8 supergravity, where her work on conserved quantum numbers served as a basis for the next hot idea coming around the bend - Superstring Theory.

These were also the years where she was called upon to lend her voice to decisions being made in the United States about the future of particle physics. The US had been steadily falling behind the particle work being done in Europe, and physicists were pinning their hopes on a new generation of super-colliders to not only put the US in the vanguard of particle researchers once again, but secure work for the emerging generation of physics hopefuls. Debate at the time centered on whether funds should be poured into a long-delayed new collider at Brookhaven, or whether instead those resources should be diverted to the proposed Superconducting Super Collider (or SSC) to be built in Texas. Gaillard realized, from a scientific point of view, that the Brookhaven project was doomed - too little, too late - and that all hope should be pinned on the SSC, and helped bring that decision about. She could not have known that Congress would, in an act of politically motivated grandstanding, later stab the SSC in the back, cutting off its funding, and compelling the US physics community to rely on CERN’s upcoming Large Hadron Collider as a depository for its physics talent, while it waited for happier political days.

Gaillard and Murray Gell-Mann

With the death of the SSC, Gaillard made the reasonable decision to turn her research towards physics of such a scale that no collider, existing or likely to exist anytime soon, could probe it, allowing her mind to take the place of the machinery that would not be forthcoming, while also reaching into the field of science education, so that the next generation might be more scientifically literate than the last and make more informed decisions about public science policy. She served on the National Science Board from 1996 to 2002, and in 2015 wrote her memoirs, A Singularly Unfeminine Profession, sharing her story of the gender prejudice and institutional inertia she had fought against for half a century.

Mary Katharine Gaillard passed away on May 23, 2025, in her Berkeley home, at the age of 86. 

FURTHER READING:

There are several interviews with Gaillard towards the end of her life available online. Her book, A Singularly Unfeminine Profession, is a gold mine of material about her scientific and professional life, but purposefully steers clear of giving the reader much insight about her domestic life beyond the tactical challenges it presented. It is also meant to be an introduction into particle physics intended for a general audience, and while there are some attempts throughout the book at building up particle physics from a foundational level, more often it’s the case that Gaillard can’t help herself, and her enthusiasm carries her into specialized vocabularies and theories that a non-physics reader will probably find completely opaque. If you’re looking to understand more about the work that she did from an actual beginner’s level, you can’t go much wrong with Frank Close’s Particle Physics: A Very Short Introduction. I’ve put it in the hands of several students, and it always achieves its end of opening new vistas to them. So, if you’re a Motivated Newcomer to physics, I would suggest reading Close first, and then reading Gaillard, and you’ll get a lot more out of it!