Кварк-глюоны плазмын шингэн төлөв дэх кваркийн долгионыг анх удаа илрүүллээ

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

Quarks Leave Wakes in Primordial Plasma

A CERN team led by MIT physicists has now found clear evidence that quarks generate wakes as they travel through this plasma, much like a duck creating ripples as it moves across water. The observations provide the first direct evidence that quark-gluon plasma responds to fast-moving particles as a unified fluid, producing waves, splashes, and swirling motion instead of simply behaving as a collection of independently scattering particles.

“It has been a long debate in our field, on whether the plasma should respond to a quark,” says Yen-Jie Lee, professor of physics at MIT. “Now we see the plasma is incredibly dense, such that it is able to slow down a quark, and produces splashes and swirls like a liquid. So quark-gluon plasma really is a primordial soup.”

Lee and his colleagues developed a new method for detecting these quark wakes. They plan to use the technique on additional particle collision data to search for more examples and study them in greater detail.

By measuring how large the wakes become, how quickly they travel, how far they extend, and how long they take to fade, scientists may be able to determine important properties of quark-gluon plasma. Those measurements could also offer clues about how the plasma behaved during the first microseconds after the universe began.

“Studying how quark wakes bounce back and forth will give us new insights on the quark-gluon plasma’s properties,” Lee says. “With this experiment, we are taking a snapshot of this primordial quark soup.”

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 (CMS), one of the general-purpose particle detectors at CERN’s Large Hadron Collider. Researchers used the CMS experiment to identify signs of quark wakes in this study. The open-access findings appear in Physics Letters B.

The Universe’s First Liquid

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

Scientists have also described QGP as a near-“perfect” liquid. In this unusual state, individual 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. Burden Professor of Physics at MIT, and his collaborators. Known as the hybrid model, it predicts that quark-gluon plasma should react like a fluid when energetic particles travel through it.

According to the model, a fast-moving jet of quarks should disturb the surrounding plasma and leave a wake behind, causing the material to ripple and splash.

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 smashed together. The collisions briefly create tiny droplets of primordial matter that usually survive for less than a quadrillionth of a second.

Researchers must effectively capture a snapshot of that fleeting moment and use the resulting particle patterns to reconstruct the properties of the quark-gluon plasma.

Why Quark Wakes Were Difficult to See

Previous searches for quark wakes often focused on pairs consisting of a quark and an “antiquark.” Antiquarks are counterparts to quarks whose certain properties have the same magnitude but opposite signs.

When a quark moves rapidly through the plasma, an antiquark may be produced traveling at the same speed in the opposite direction. Scientists therefore searched for quark and antiquark pairs, expecting both particles to produce detectable wakes in the surrounding plasma.

That approach created a major problem.

“When you have two quarks produced, the problem is that, when the two quarks go in opposite directions, the one quark overshadows the wake of the second quark,” Lee says.

Lee and his colleagues realized that the wake from a single quark would be much easier to identify if there were no second quark creating an overlapping disturbance.

“We have figured out a new technique that allows us to see the effects of a single quark in the QGP, through a different pair of particles,” Lee says.

Using Z Bosons as a Wake Tag

Instead of searching for quark and antiquark pairs after lead ion collisions, the researchers looked for events in which one quark traveled through the plasma in nearly the opposite direction from a “Z boson.”

A Z boson is a neutral elementary particle associated with the weak force. It interacts very little with the surrounding plasma, making it useful as a clean reference point. Z bosons also appear at a distinctive energy, which makes them relatively easy for physicists to identify.

“In this soup of quark-gluon plasma, there are numerous quarks and gluons passing by and colliding with each other,” Lee explains. “Sometimes when we are lucky, one of these collisions creates a Z boson and a quark, with high momentum.”

When such a collision occurs, the quark and Z boson should fly away from each other in opposite directions. The quark can disturb the plasma and produce a wake, while the Z boson should pass through without significantly affecting the material around it.

That means any ripples appearing in the plasma on the quark’s side can be attributed to the quark itself.

Working with Professor Yi Chen’s group at Vanderbilt University, the researchers realized they could use Z bosons as a “tag” for locating and measuring wakes created by individual quarks.

Wakes Found Among Billions of Collisions

The team analyzed data from heavy-ion collisions at the Large Hadron Collider. Among 13 billion collisions, they identified roughly 2,000 events in which a Z boson was produced.

For each of those events, the researchers mapped how energy was distributed throughout the short-lived quark-gluon plasma. They repeatedly found fluid-like patterns of splashes and swirling motion in the direction opposite the Z boson.

Because the Z boson itself barely interacts with the plasma, the researchers could attribute these wake patterns directly to individual quarks traveling through the material.

The observed wakes also matched predictions from Rajagopal’s hybrid model. The results indicate that quark-gluon plasma really does respond collectively like a liquid when energetic particles pass through it.

“This is something that many of us have argued must be there for a good many years, and that many experiments have looked for,” says Rajagopal, who was not directly involved with the new study.

“We’ve gained the first direct evidence that the quark indeed drags more plasma with it as it travels,” Lee adds. “This will enable us to study the properties and behavior of this exotic fluid in unprecedented detail.”

This work was supported, in part, by the U.S. Department of Energy.