Owner of a Lonely Quark: The High Temperature Quantum Chromodynamics of Ágnes Mócsy (Women in Science 68)
When planning a trip to the universe’s first millionth of a second of existence, there are really only two things the canny traveler needs to keep in mind:
(1) Don’t pack woolens.
At 4 000 000 000 000 degrees Celsius, or some three hundred thousand times hotter than the center of the sun, prudence dictates breathable fabrics. Basically, just chalk the whole thing up as a shorts and sandals affair.
(2) Bring a book.
None of your favorite things are there.
In fact, none of the things your favorite things are made of, like molecules, are there.
In double fact, none of the things those molecules are made of, like atoms, are there.
To be completely honest, at these temperatures the only chaps around are free-roaming quarks and gluons in a flowing plasma state, and the only topic they have to talk about is that really singular thing that just happened a millionth of a second ago and that they just won’t stop going on about.
So, bring a book.
That first millionth of a second is a strange realm populated by impossible things, and for the longest of whiles, we only had theory to feel our way through its mysteries. Within the last three decades, however, particle accelerators have grown to a size and power to allow us to smash heavy ions into each other with enough energy to recreate conditions of the early universe, with results that continue to illuminate and challenge, proving once again thatnuclear physics has miles to go before it sleeps.
These experiments are massive international ventures involving the collaboration of hundreds or even thousands of scientists, technicians, and engineers whose workaday business it is to probe the earliest moments of existence and analyze the behavior of the quarks and gluons that, upon settling, formed the basis for our friends, the proton and neutron. Among the most fascinating people in that small army is Ágnes Mócsy, a Romanian born American quantum chromodynamicist studying quarkonia (matter composed of a heavy quark and its anti-quark, like Charmonium or Bottomonium) and the physics of the early universe, who is also an advocate for greater female and race representation in physics, a fashion guru, a documentary film-maker, and science literacy spokesperson.
That might sound an unreasonable number of hats for any one human being to wear at the same time, but juggling interests and varied intellectual disciplines has been part of Mócsy’s life from the start. She was born into a Hungarian minority family in Transylvania during the time of Ceausescu’s dictatorship, and spent her entire youth and adolescence under that regime, experiencing an education consisting of state-mandated classes that makes the head reel to contemplate: math, physics, geography, history, biology, material science, drawing, political science (through the obligatory Marxist interpretive perspective), gym, music, Hungarian literature and grammar, Romanian literature and grammar, English, German, and Latin.
It’s something to bring out the next time a high schooler starts complaining about having to take two whole years of a foreign language in high school.
Always a math enthusiast, Mócsy didn’t fall in love with physics until high school when a charismatic teacher presented the material in a fun and engaging way that showed its potential as a life discipline. When she was eighteen, Ceausescu fell from power and soon thereafter Mócsy began her amazing international career, getting her Master’s degree in Norway, popping over to Minneapolis for a PhD, then around to Copenhagen for post-doctoral work at the famous Niels Bohr Institute and Frankfurt’s Goethe University, before ending up in New York at the Brookhaven National Laboratory. In science, you go where the kind of work you’re interested in is being done, which is one of Mócsy’s favorite aspects of being a scientist – the pure internationalism of the effort, moving and living in different places with different people, and having colleagues from the world over.
Her specialty, as it emerged, dealt with the problems surrounding the puzzling behavior of quarks. Under even the most normal of circumstances, their antics are just plain bizarre. Take the phenomenon of confinement. At energies characteristic of our universe as it stands now, quarks are not found in isolation. They are tightly bound into composite particles by the Strong Force, which is carried by particles called gluons. If two quarks are separated, they form a gluon bridge between them that carries a force of around ten thousand Newtons. If you manage somehow to separate them further in spite of that force, rather than splitting into two separate quarks, they create quark and antiquark partners for themselves out of nothingness, making four quarks where before there were two. Creating a partner out of nothing actually represents a lower energy investment than lengthening the gluon bridge, so the quarks figure, “Cool, we’ll do that, then.”
Another of the strange things that quarks get up to all the time is wrapped up in the notion of color screening and anti-screening. A quark is constantly circled by a cloud of virtual quark-antiquark pairs created from the currency of quantum uncertainty (because the universe doesn’t quite know your precise velocity, it doesn’t quite know your precise energy, and if you’re a crafty particle, you can use that wiggle room to create “virtual” masses to follow you around as a kind of entourage).
That cloud tends to obscure what “color” the quark inside is (remember that quarks come in three charges, which we whimsically refer to as colors: red, green, or blue). However, at the same time, there’s a field of virtual gluons around the quark, the effect of which is to augment and change the color. The ability of virtual quark-antiquarks to obscure color is called Screening, and of gluons to augment and change color is Anti-Screening. To reiterate, a bunch of particles that oughtn’t exist is in constant competition with a different group of particles that also oughtn’t exist to control the perceived color of a quark.
It’s a strange mental world to live in, where common sense often checks itself at the door in order to let King Math reign to the fullest of its reasoned fancy, and where reality serves up the unexpected at a brisk and exciting rate to its students. To get an idea for the bracing liveliness of the field, here is an excerpt from “Heavy Quarkonium: Progress, Puzzles, and Opportunities”, a 2011 paper which expresses equal parts challenge, consternation, and giddiness at what has been discovered and what is left to do in the future of quarkonium studies:
Heavy quarkonium spectroscopy examines the tableau of heavy-quark bound states, thereby providing the starting point for all further investigations. Which states exist? Why? What are their masses, widths, and quantum numbers? Which states should exist but have not yet been observed? Does Quantum Chromodynamics fully explain the observed terrain? If not, why?… Have we observed mesonic molecules? Tetraquarks? Quark-gluon hybrids? How would we know if we had? How many of the new states are experimental artifacts?
I find it immensely exciting to hear a scientific discipline engage with its subject in this way, with wonder and a palpable anxiousness to have a crack at thorny problems that confound all explanation, and create new methods to determine what, in fact, we know, and why. Mócsy’s work in quantum chromodynamics (the field of physics that deals with the strong interactions created by the exchange of gluons by quarks) has helped illuminate further strangeness in the world of the quarks by demonstrating that confinement, that to be frank rather desperate inability of quarks to exist singly, breaks down above a certain temperature, such as that of the universe’s first millionth of a second. At those energy levels, quarks become unglued and roam as free agents in a quark-gluon plasma that flows more easily than water.
Mócsy has investigated the difficulties of using quarkonia as a kind of thermometer to gauge a given heavy ion collision’s temperature. It’s an elegant idea: at higher temperatures, even the smallest, most tightly bound forms of quarkonia start to dissociate. By looking at the abundance of different sizes of quarkonia, then, you should be able to tell how hot your plasma got, providing a way of measuring temperature at heats where, let’s just say, mercury thermometers are not entirely at their best. That research has detailed the contributions of color screening and other effects to the separating of quarks, and resulted in a more nuanced sense of high-temperature quarkonium binding. She continues to investigate new questions centered on how the universe and matter as we know it resulted from that curious quark-gluon plasma state.
Meanwhile, there is the massive rest of Mócsy’s life, because having a hand in creating and studying the hottest substances yet created by humanity is just one of many things she does. Her most insistent advice to people looking for a career in science is to, “get attached to more senior experienced scientists, to learn what research is really about – something that one can learn mostly by doing it and doing it next to someone experienced is key.” She also emphasizes the importance to engage with the community, both the scientific community, by attending and speaking at seminars whenever the opportunity arises (yes, it can be scary, but it is also rewarding and fun), and the larger community of humans by communicating effectively the work and discoveries of science in multiple formats. “Physics is not just for physicists,” she insists, “Physics is accessible to everyone if we make it accessible. I love research, and I enjoy making people fall in love with science.” Mócsy’s films (one of which you can find here ) advance that agenda with a flair for artistic communication. Her most recent film, “gets Behind The Science Scene and showcases the people and their very human and thus quite familiar feelings as they develop and advance new ideas based on evidence and curiosity, ideas that turn into full blown state-of the-art science fields.” At the time of this writing she is placing the finishing touches on this documentary before sending it off to film festivals and release.
“We tend to make people choose whether they are arsty or science-y and in a way I wish we would not do that. It’s not an either/or question,” she explains. “There really is no one way for a person to be science-y. Scientists can be many different things. All it takes to be a scientist is curiosity, hard-work, and developing a way of thinking about things carefully and clearly.”
Mócsy is, then, part of the long tradition of women in science who marry cutting edge science with creative expression, stretching from mathematician-novelist Sofia Kovalevskaya through astronomer-artist Cecilia Payne-Gaposchkin to physicist-musician Fabiola Gianotti. Life is long, and science wide, and there is room in both for humans to be whole if they carry the courage of their passions and keep before them the examples of our very best truth-seekers. As long as there are people like Ágnes Mócsy about, we won’t be lacking in the latter.
FURTHER READING: Quantum Chromodynamics is a young and exciting field that hasn’t quite penetrated the public excitement in a manner commensurate with the magnitude of its findings. To get a running start at it, you could start with Richard Feynman’s classic and accessible volume QED on Quantum Electrodynamics, the branch of physics he co-founded, and which deals with the interactions between matter and photons. With that, you’ll be ready to appreciate how strange QCD really is, and what a magnificent difference results from the seemingly small fact that gluons interact with each other but photons don’t (at least not directly). Then you can take a crack at this!