In the fraction of a second following the Big Bang, temperatures soared to trillions of degrees, creating a chaotic, intensely hot soup of elementary particles. In this state, known as quark-gluon plasma (QGP), quarks and gluons raced around at nearly the speed of light. This primordial material existed for only a few millionths of a second before cooling rapidly, allowing those fundamental building blocks to combine into the protons, neutrons, and other particles that populate the universe today.
To investigate the ingredients that filled the young universe, physicists at CERN’s Large Hadron Collider in Switzerland are recreating quark-gluon plasma in modern laboratories. By colliding heavy ions at nearly the speed of light, researchers can briefly separate quarks and gluons, producing tiny, fleeting droplets of the exact same kind of matter that existed during the cosmos’s first microseconds.
Now, a CERN research team led by physicists from the Massachusetts Institute of Technology (MIT) has found clear, direct evidence that quarks generate wakes as they travel through this plasma. The phenomenon is much like a duck creating ripples as it glides across water. These observations provide the first direct proof that quark-gluon plasma responds to fast-moving particles as a unified fluid, producing waves, splashes, and swirling motion rather than 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, a 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 an innovative new method for detecting these elusive quark wakes. The team plans to apply the technique to additional particle collision data to search for more examples and study them in greater detail.
By measuring the physical dimensions of the wakes—including how large they become, how quickly they travel, how far they extend, and how long they take to fade—scientists hope to determine crucial properties of quark-gluon plasma. In turn, these precise measurements could offer unprecedented clues about how the plasma behaved during the very 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, an international collective of particle physicists who conduct and analyze experiments using the Compact Muon Solenoid (CMS). The CMS is one of the massive, general-purpose particle detectors situated along CERN’s Large Hadron Collider ring. Researchers utilized the CMS experiment to identify the subtle signs of quark wakes detailed in the study, which has been published as open-access findings in Physics Letters B.
Scientists believe quark-gluon plasma was the first liquid in the universe. It holds the distinction of being the hottest liquid ever known, reaching temperatures of several trillion degrees Celsius during its ephemeral existence.
Furthermore, researchers have frequently described QGP as a near-"perfect" liquid. In this highly unusual state of matter, individual quarks and gluons appear to move together as an exceptionally smooth fluid exhibiting almost no friction.
This deep theoretical understanding stems from numerous past experiments and advanced analytical studies. One particularly influential framework was developed by Krishna Rajagopal, the William A. M. Burden Professor of Physics at MIT, alongside his collaborators. Known in the physics community as the hybrid model, this theoretical framework predicts that quark-gluon plasma should react like a continuous fluid when energetic particles travel through its dense interior.
According to the model, a fast-moving jet of quarks should physically disturb the surrounding plasma and leave a turbulent wake behind it, forcing the material to ripple, splash, and swirl.
For years, experimental physicists have searched tirelessly for definitive evidence of these wakes at the Large Hadron Collider and other high-energy particle accelerators around the globe. During these complex operations, heavy atomic ions such as lead are accelerated to velocities approaching the speed of light and smashed together head-on. These catastrophic collisions briefly generate microscopic droplets of primordial matter that typically survive for less than a quadrillionth of a second.
To learn about the material, researchers must effectively capture a high-speed snapshot of that fleeting moment, utilizing the resulting scattering patterns of secondary particles to mathematically reconstruct the properties of the underlying quark-gluon plasma.
Historically, previous searches for quark wakes faced significant technical hurdles because they frequently focused on particle pairs consisting of a quark and an "antiquark." Antiquarks are the direct counterparts to quarks, possessing properties of the same magnitude but with opposite mathematical signs.
When a quark moves rapidly through the dense plasma, an antiquark may be simultaneously produced, traveling at the same high speed in the exact opposite direction. Consequently, scientists initially searched for paired combinations of quarks and antiquarks, expecting both particles to leave detectable wakes in the surrounding plasma medium.
That standard approach, however, created a major analytical problem that stymied researchers for years.
"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 explains.
Realizing the limitations of looking at paired jets, Lee and his colleagues deduced that the wake generated by a single, solitary quark would be vastly easier to identify if there was no secondary quark creating an overlapping disturbance in the plasma.
"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.
Rather than searching for traditional quark and antiquark pairs following high-energy lead ion collisions, the researchers adjusted their strategy. They looked specifically for events in which a single quark traveled through the plasma in nearly the opposite direction from a neutral elementary particle known as a "Z boson."
A Z boson is associated with the fundamental weak nuclear force. Crucially, it interacts very little with the surrounding quark-gluon plasma, making it an exceptionally clean reference point for physicists. Additionally, Z bosons appear at a highly distinctive energy level, which renders them relatively easy for researchers to reliably identify amidst the debris of particle collisions.
"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 rare collision occurs, the newly minted quark and Z boson fly away from each other in nearly opposite directions. While the quark plunges into the plasma, disturbing the material and producing a distinct wake, the Z boson passes straight through the medium without significantly affecting or interacting with the matter around it.
This dynamic means that any hydrodynamic ripples appearing in the plasma on the quark’s side of the collision event can be definitively attributed to the quark itself, uncorrupted by competing signals.
Collaborating with Professor Yi Chen’s research group at Vanderbilt University, the MIT team realized they could effectively utilize Z bosons as a reliable "tag" for locating, isolating, and measuring wakes created by individual quarks.
To test this novel technique, the team analyzed vast quantities of data generated by heavy-ion collisions at the Large Hadron Collider. Sifting through a massive dataset of 13 billion individual collisions, the researchers successfully identified roughly 2,000 specific events in which a Z boson was produced alongside a high-energy quark.
For each of those isolated events, the researchers meticulously mapped how energy was distributed throughout the short-lived volume of quark-gluon plasma. Time and again, they identified clear, fluid-like patterns of splashes and swirling motion localized precisely in the direction opposite the detected Z boson.
Because the accompanying Z boson barely interacts with the plasma at all, the researchers could confidently attribute these distinct wake patterns directly to individual quarks traversing the exotic material.
Furthermore, the observed wakes matched theoretical predictions generated by Rajagopal’s hybrid model with striking accuracy. The results provide robust empirical confirmation that quark-gluon plasma truly responds collectively like a liquid when energetic particles pass through its bulk.
"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 in 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 scientific endeavor was supported, in part, by funding from the U.S. Department of Energy.
