Artist’s illustration of a “wake” created in quark-gluon plasma. (Image: Larisa Barannikova) Listen to story summary

Over the course of 20 years, physicists have confirmed that a primordial state of matter known as quark-gluon plasma behaves like a liquid. Now, researchers at the University of Illinois Chicago have solidified that idea by observing a long-predicted effect of fluid behavior: that particles leave a wake as they move through the plasma, like the wake left by a boat crossing a lake.

The discovery, published in Physical Review Letters by the CMS Collaboration at CERN’s Large Hadron Collider, is the first observation of a “jet diffusion wake” in quark-gluon plasma. It offers new insights into the properties of the matter that filled the universe immediately after the Big Bang.

Quark-gluon plasma is believed to have existed in the moments right after the Big Bang, before the universe cooled enough for matter to form atoms, stars and planets. Scientists recreate quark-gluon plasma by smashing heavy ions together at nearly the speed of light inside particle accelerators such as the Large Hadron Collider.

Researchers first discovered quark-gluon plasma more than two decades ago at the Relativistic Heavy Ion Collider in New York and studied it further at the Large Hadron Collider in Switzerland. One of the biggest surprises along the way was that the plasma behaved not like a gas, as many physicists had expected, but like an almost-perfect liquid with remarkably low viscosity. Yet one thing remained elusive.

“If it is a fluid, there should be a wake in it from fast-moving particles, just like on a lake,” said Olga Evdokimov, UIC professor of physics and a member of the CMS Collaboration, an international team of several thousand scientists from over 60 countries. “You wouldn’t picture a boat moving on water with nothing trailing behind. So that was sort of a missing piece of this grand puzzle.”

Observing that wake presented a major challenge because quark-gluon plasma exists for only an instant.

“The plasma that we create in these heavy-ion collisions is very, very short-lived,” Evdokimov said. “It explodes, and we’re really just trying to dissect what happened from the particles that are already produced after it ceases to exist.”

Display of a lead-lead collision, which produced two back-to-back jets (indicated by orange cones), recorded by the CMS experiment. (Photo credit: CMS Collaboration)

To reveal the wake, researchers used particle jets generated during collisions as probes of the plasma. The technique can be compared to medical imaging, Evdokimov said.

“We used jets for many years for understanding the plasma properties. It’s like an X-ray machine,” she said.

For Evdokimov and the UIC team, the results represent the culmination of a long-running effort.

“My group, for close to 20 years, has been trying to observe this wake,” she said. “At some point, we didn’t have enough data. We didn’t have the right idea. We were looking in the wrong place.” The breakthrough came after new theoretical studies, and was spearheaded by Evdokimov, her postdoctoral researcher Raghunath Pradhan, and the UIC team.

The observation will help physicists refine models of how quark-gluon plasma responds when energetic particles move through it, advancing efforts to understand the conditions that existed at the dawn of the universe.

UIC researchers are already looking ahead to future studies that will use increasingly precise data from the Large Hadron Collider to probe quark gluon plasma’s properties.

“We are so thrilled to have it making waves, pun intended,” Evdokimov said.