A few weeks ago, I learned of the passing—in May of this year at the age of 86—ofMary K. Gaillard, a titan of theoretical particle physics, the field that describes and predicts what happens to matter inside atoms and what exists there. Today we understand that there is a force that holds nuclei together, and another that breaks them apart. And these forces involve the exchange of different elementary particles. These phenomena are described by the so-called Standard Model of Particle Physics. But consolidating this knowledge has required hundreds of contributions from various scientists. Mary K. Gaillard was a key scientist in understanding the properties of the most basic constituents of nuclei: the so-called quarks. And one in particular, a truly charming one. The so-calledcharm(c) quark.
We had known about the possibility of quarks since the 1960s, thanks to physicists George Zweig and Murray Gell-Man, who independently predicted the existence of quarks to explain the different quantum numbers (such as electric charge) of particles composed of quarks. Today we know there are six types of quarks in total. But Murray’s original model had only three (the three we now know to be the lightest): the so-calledup (u),down (d), andstrange (s) quarks.It took more than a decade to convince us that a fourth, more massive quark existed: thecharm quark.
As the existence of quarks as real particles—and not merely mathematical constructs—became increasingly established, there was some resistance to the idea that these particles could have fractional electric charges. This was because no such charge had ever been observed. Today we understand that quarks always travel together (they are confined to the nuclei of atoms; in fact, a single quark has never been measured). Protons, for example, are made of three quarks: twoupquarks and onedown quark. The fractional electric charges of the quarks within the proton combine to give a total positive electric charge of +1.
Mary K. Gaillard, overcoming resistance, began studying some very strange particles called kaons. They were literally called“strange particles,” since their production in pairs involved a new quantum number called“strangeness.” Kaons are composed of astrangequark and ananti-down quark, giving neutral kaons a net electric charge of zero. Neutral kaons can then decay into other elementary particles, such as a muon (with a charge of -1) and an antimuon (with a charge of +1, the muon’s antiparticle), while respecting the conservation of electric charge in particle interactions.
Mary K. Gaillard became a world-renowned authority on kaon physics. The theory at the time predicted that the decay of neutral kaons into muons and antimuons would occur with a low probability only if a fourth quark—thecharm quark—existed, one that had the same electric charge as theupquark. And this probability depended not only on the mass of the charm quark, but also on the mass of the up quark. This is because quarks do not interact solely with each other to form protons or kaons. They can also interact through the exchange of other massive particles called W bosons. And these can also transform or decay into different types of quarks. Therefore, how frequently this decay occurs depends on both masses—both the mass ofthe up quarkand that ofthe charm quark. Experimentally, it was found that this decay was indeed suppressed.
But Mary K. Gaillard dared to ask the following question: Why isn’t the decay of a neutral kaon into two photons—if it is governed by the same theory—equally suppressed in experiments?This decay also preserves the electric charge of the quarks, since the photon has no electric charge. And, ifcharm existed, it was theoretically expected that its probability would also be small, comparable to the decay of kaons into muons and antimuons. But it was larger. This was concerning, as it could severely call into question the construction of the theory at the time, which relied heavily on the predictions made by this suppression mechanism (known today as the“GIM mechanism”in particle physics).
Solving this mystery regarding kaon decays—and, even more so, convincing herself that the theoretical foundations of the time were not flawed—led to Mary K. Gaillard’s arduous and systematic work in the study ofcharmed kaon physics. Work that, particularly in that era of particle physics, depended indirectly on experimental observations, so one had to be very alert to every new result.
Gaillard's work explainswhy the decay of kaons into a muon and an antimuon is extremely suppressed in nature, and why the two-photon decay is not. In the first case, the decaydepends on the mass difference between theupandcharm quarks. This difference had to be much smaller than the mass of the W boson (which was involved in the decay), and at the time it was assumed (though not yet known) that its mass had to be much greater than the mass of the proton. The study of the second decay leads to the understanding that the mass of the up quark had to be much smaller than the mass of the charm quark. Mary K. Gaillard (along with physicists Benjamin Lee and Jonathan Rosner) calculated this difference in 1975, concluding that the charm quark’s mass must be around 1.5 GeV (in units of energy). In other words, Mary K. Gaillardpredicted the mass of thecharmquark three months before the charm quark was discovered!
In this study of kaon decays, it is the difference between the particles’ masses that matters—not just the specific value of each mass in isolation, but how different they are from one another. This really catches my attention; the underlying explanation is elegant. To draw an analogy,it is as if nature knew that what truly shapes the melody of a song(decay frequency) depends on the distance between the notes, and not on each note (mass) separately.
Gaillard was able to see what his contemporaries failed to see, to ask questions that others did not dare to ask, and to study strange phenomena and particles that did not interest everyone. Later in his career, his contributions included predicting the mass of a new quark, thebottom (b)orbeautyquark, together with M.S. Chanowitz and J. Ellis, as well as identifying, together with J. Ellis and G. Ross, that a gluon—the particle responsible for mediating strong nuclear interactions between quarks—could be observed as a three-jet event in electron-positron collisions. This is indeed how the gluon was subsequently discovered.
In her autobiography, Mary K. Gaillard summarizes her numerous discoveries, honestly detailing the contexts in which they were made. Thebookis titled*A Singularly Unfeminine Profession*, published in 2015. The title stems from her hearing those words from a neighbor, in response to her saying she wanted to study physics. I came across her book at the CERN bookstore in 2018. The title immediately caught my eye. I reread it before writing these words.Mary K. Gaillard’s legacy extends far beyond her science. It reminds us that passions do not discriminate: they choose us.
