LHC's latest heavy-ion run recreates conditions of early universe in quark-gluon plasma

University of Illinois Professor Anne Sickles leads ATLAS’s Heavy Ion Working Group

Siv Schwink
for Illinois Physics

For a few millionths of a second shortly after the big bang, the universe was filled with an extremely hot, dense soup of elementary particles moving at near the speed of light. The quark-gluon plasma (QGP), so called because it was predominantly made up of quarks and gluons, can be recreated and studied only in particle accelerators, in the high-energy collisions of heavy ions such as gold or lead. 

In early November, the Large Hadron Collider (LHC) at CERN in Switzerland began its first lead-lead run since 2015, the final run of this season before a long shutdown for upgrades to the accelerator. Beams of lead nuclei (having been stripped of their electrons) were collided head-on at an energy of 5.02 TeV during a run of three-and-a-half weeks. 

Lead nuclei comprise 208 protons and neutrons, which in turn are made up mostly of quarks and gluons. When the 416 protons and neutrons of two lead nuclei are smashed together, they form a tiny fireball that “melts” everything into a very fleeting QGP, which scientists have learned is a nearly perfect fluid having small viscosity. But the fireball cools very quickly, and the quarks and gluons recombine into ordinary matter, which speeds away in all directions. This collision debris contains particles such as pions and kaons, which are made of a quark and an antiquark; protons and neutrons, made of three quarks; and even copious antiprotons and antineutrons, which may combine to form the nuclei of antiatoms as heavy as helium.

Scientists on the four experiments at the LHC—ALICE, ATLAS, CMS and LHCb—study the QGP to shed light on some of the most fundamental unanswered questions about matter. Each experiment uses its own detectors to trace, plot out, and selectively record the immediate aftermath of the collisions, looking at the QGP and the subsequent decay paths of the subatomic particles. Only a small fraction of these collision events can be recorded, however, and deciding what to record and what to throw out has everything to do with what specific questions the researchers are trying to answer.

The ATLAS group is collecting data at a rate of 4.5 GB per second while the accelerator is actively running, which it does for about 50 percent of the total scheduled runtime. Professor Anne Sickles of the University of Illinois at Urbana-Champaign is a co-convener of the ATLAS experiment’s Heavy Ion Working Group, alongside Professor Martin Spousta of the Charles University in Prague. As such, Sickles and Spousta manage the ATLAS experiment’s heavy-ion analytical strategy, which in turn informs its “trigger strategy”—what collision data are automatically preserved for analysis.

“Most of what the LHC delivers are proton-proton collisions,” Sickles points out. “We are the only working group in the ATLAS experiment that has a distinct data set. That puts us in the unique position of deciding what kind of triggers we take. The working group has grown and matured since the last heavy-ion run three years ago, and we’re asking more complicated questions now that build on what we learned then. Martin and I coordinated among the 60 members of our working group—scientists from institutions around the globe—to devise a trigger strategy to maximize our physics opportunities within this short run. The trigger plans balance our physics goals and take full advantage of the tremendous capabilities of ATLAS for selecting and recording data.”

During the heavy ion run, two students and a postdoc from Sickles’ group, as well as Sickles herself, were at CERN, validating the data as they were being recorded. Sickles’ research group in Illinois is particularly interested in jets—sprays of elementary particles having tremendous momentum. Jets occur when one quark or gluon within a nucleus collides head-on with another quark or gluon within the other nucleus.

Sickles’ group is especially interested in what happens to the QGP when lighter quarks in the jets scatter through it.

“There are six different kinds of quarks,” explains Sickles. “Light quarks—the up, down, and strange quarks—are most abundant, and we have good data on these, but there are still unresolved questions. For example, if we turn up the momentum of these light quarks, make them go faster and faster through the QGP, one might imagine their eventually moving so fast they don’t even notice they are going through the QGP. But we haven’t seen that—as fast as they are going, we still see big effects, and that’s puzzling.” 

Scientists in the ATLAS Heavy Ion Working Group are also interested in looking at more massive quarks and what happens when they move very quickly through the QGP.

“Charmed quarks are heavier and less abundantly produced,” she continues. “They are challenging to study for that reason and because it’s hard to trigger on them. We’ve devised a trigger strategy that simply involves keeping a whole lot more events, and we will filter out the charmed quarks later.”

The ATLAS working group is also interested in properties exhibited when the heavy ions fail to collide, passing by one another in a near miss.

“As a program, we are also very interested in what happens when the nuclei don’t quite hit each other,” Sickles comments. “If you think about these nuclei, lead is big—it’s at the bottom of the periodic table. We’ve stripped away all of the electrons, so each nucleus has a positive 82 charge—in other words, there are 82 protons in each lead nucleus. The nuclei each have huge electromagnetic fields and when they pass closely by each other, they create interesting interactions that couldn’t happen anywhere else.”

One surprising example of this is the scattering of two photons into two photons, a finding the ATLAS group published in August 2017 in the journal Nature Physics.

Sickles explains, “We don’t think of photons as something that would interact with themselves, but we found they can. In a near miss event, the lead nuclei pass by each other and shed photons—because electromagnetic interactions happen via the exchange of photons. And every now and then, two photons collide and scatter. Our result was the first direct evidence of the scattering of two photons. We observed about 10 events using the previous data. With these new data we are interested in following up on this and improving the precision of the previous measurement.”

“What’s also interesting, though a little further afield,” Sickles adds, “this could be a way to look for physics beyond the standard model. In the heavy-ion program, our headlining program is making the QGP and studying its properties. However, there’s been interest from many people in this idea, both within the Heavy Ion Working Group and outside of it. Photons colliding and scattering: maybe they could create dark matter, maybe they create something else. We don’t know. It’s possible, if we had more data, we could find something more exotic. I don’t have high expectations in terms of this run, but with future increases in luminosity, I expect our program will expand its efforts along these lines in the search for new physics.”