At CERN’s Large Hadron Collider in Switzerland, physicists are recreating quark-gluon plasma to study the matter that filled the young universe. By colliding heavy ions at nearly the speed of light, researchers can briefly separate quarks and gluons, producing small amounts of the same type of matter that existed during the universe’s first microseconds.

Quarks leave wakes in the early universe’s plasma

A CERN team led by MIT physicists has found clear evidence that quarks create wakes as they move through quark-gluon plasma—much like a duck creates ripples as it moves across water.

The observation provides the first direct evidence that quark-gluon plasma responds to fast-moving particles as a unified fluid. Instead of behaving like a collection of independently scattering particles, the plasma generates waves, droplets, and swirling motion.

“There has been a long debate in our field about whether plasma should react with quarks,” says Yen Jie Li, a physics professor at MIT. “We found that the plasma is incredibly dense, slowing down the quarks and producing droplets and vortices like a liquid. So the quark-gluon plasma is really a primordial soup.”

Li and his colleagues developed a new method for detecting quark trails. They plan to apply the technique to additional particle-collision data to find more examples and study the wakes in greater detail.

By measuring the size of a wake, how quickly it travels, how far it extends, and how long it takes to dissipate, scientists may be able to determine important properties of quark-gluon plasma. These measurements could also provide clues about how the plasma behaved during the universe’s first microseconds.

“Studying how quark wakes travel back and forth will provide new insights into the properties of quark-gluon plasmas,” Li said. “In this experiment, we’re taking a snapshot of this primordial quark soup.”

The hottest and first liquid in the universe

Quark-gluon plasma is believed to have been the first liquid in the universe. It was also the hottest liquid ever known, reaching temperatures of trillions of degrees Celsius during its brief existence.

Scientists describe QGP as a nearly “perfect” liquid. In this unusual state, quarks and gluons appear to move together as an exceptionally smooth fluid with almost no friction.

This understanding comes from numerous experiments and theoretical studies. One influential model was developed by Krishna Rajagopal, the William A. M. Baden Professor of Physics at MIT, and his collaborators. The model predicts that quark-gluon plasma should respond like a fluid when high-energy particles pass through it.

According to the model, jets of fast-moving quarks should disturb the surrounding plasma and leave behind wakes, producing ripples and droplets in the matter.

Physicists have spent years searching for evidence of these wakes at the Large Hadron Collider and other high-energy particle accelerators. In these experiments, heavy ions such as lead are accelerated to nearly the speed of light and collided with one another. The collisions produce tiny droplets of primordial material for an extremely short time, typically less than a thousandth of a second.

Researchers must capture this fleeting snapshot and use the resulting patterns of particles to reconstruct the properties of quark-gluon plasma.

Why quark wakes are difficult to detect

Previous studies of quark wakes often focused on pairs consisting of a quark and an antiquark. Antiquarks are counterparts of quarks with properties of the same magnitude but opposite sign.

When quarks move rapidly through plasma, they can be produced alongside antiquarks moving at the same speed in the opposite direction. Scientists looked for these quark-antiquark pairs, expecting both particles to produce detectable wakes in the surrounding plasma.

However, this approach created a major problem.

“When two quarks are created, the problem is that one quark obscures the trail of the second quark as they go in opposite directions,” Li says.

Li and his colleagues realized that identifying the wake from a single quark would be easier when a second quark was not creating an overlapping disturbance.

“We have discovered a new technique that allows us to see the influence of a single quark in QGP through different pairs of particles,” Li says.

Using the Z boson to track a quark wake

Instead of looking for quark-antiquark pairs after a lead-ion collision, the researchers searched for events in which a single quark moved through the plasma in roughly the opposite direction from a Z boson.

The Z boson is a neutral elementary particle associated with the weak nuclear force. Because it interacts only weakly with the surrounding plasma, it provides a clean reference point. Z bosons also occur at distinctive energies, making them relatively easy for physicists to identify.

“In this plasma soup of quarks and gluons, many quarks and gluons are passing through and colliding with each other,” Li explains. “Sometimes, if we’re lucky, one of these collisions produces a Z boson and a quark with high momentum.”

When this happens, the quark and Z boson should fly away in opposite directions. The quark can disturb the plasma and create a wake, while the Z boson should pass through without significantly affecting the surrounding matter.

As a result, ripples appearing on the quark side of the plasma can be attributed to the quark itself.

Working with Professor Yi Chen’s group at Vanderbilt University, the researchers recognized that Z bosons could serve as “tags” for locating and measuring wakes produced by individual quarks.

Quark wakes found in billions of collisions

The research team analyzed heavy-ion collision data from the Large Hadron Collider. They identified approximately 2,000 collisions out of 13 billion that produced Z bosons.

For each event, the researchers mapped how energy was distributed throughout the short-lived quark-gluon plasma. They repeatedly found a fluid-like pattern of oppositely directed splashing and swirling motion associated with the quark and Z boson.

Because Z bosons interact only weakly with the plasma, the researchers attribute the observed wake patterns to individual quarks moving through the material.

The observed wake also matched predictions from Rajagopal’s hybrid model. The results show that quark-gluon plasma responds collectively, like a liquid, when energetic particles pass through it.

“This is something many of us have been saying must exist for years, and many experiments have been looking for,” says Rajagopal, who was not directly involved in the new research.

“We have the first direct evidence that quarks actually drag more plasma with them as they travel,” Li added. “This will allow us to study the properties and behavior of this unusual fluid in unprecedented detail.”

This research was supported in part by the U.S. Department of Energy. The study’s co-authors are members of the CMS Collaboration, a worldwide group of particle physicists who conduct and analyze experiments using the Compact Muon Solenoid, one of the major particle detectors at CERN’s Large Hadron Collider.