In the earliest fractions of a second following the birth of the universe, temperatures reached trillions of degrees, melting the cosmos into an intensely hot, roiling mixture of quarks and gluons. These elementary particles raced around at nearly the speed of light in an exotic state of matter known as quark-gluon plasma (QGP). This primordial material existed for only a few millionths of a second before cooling rapidly, allowing the liberated quarks and gluons to combine into the protons, neutrons, and other stable particles that make up the universe today.
Now, in a major advance in modern particle physics, researchers at CERN’s Large Hadron Collider in Switzerland have captured direct evidence of how this primordial soup behaves. A collaboration led by MIT physicists has discovered clear, undeniable proof that individual quarks generate physical wakes as they barrel through the plasma, behaving precisely like a duck creating ripples and waves as it moves across water.
The observations provide the first direct confirmation that quark-gluon plasma responds to fast-moving particles as a unified, cohesive fluid. Rather than acting merely as a sparse collection of independently scattering particles, the plasma produces waves, splashes, and swirling motion, confirming long-held theories about the universe’s earliest moments.
"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 entirely new method for detecting these fleeting quark wakes. They now plan to apply this sophisticated technique to a broader set of particle collision data, allowing them to search for additional examples and study the properties of the plasma in unprecedented detail.
By measuring physical characteristics such as how large the wakes become, how quickly they travel, how far they extend, and how long they take to fade, scientists hope to determine fundamental properties of quark-gluon plasma. These measurements could unlock vital clues about how the universe’s first fluid behaved during the opening microseconds of creation.
"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 massive worldwide partnership of particle physicists who conduct and analyze experiments using the Compact Muon Solenoid (CMS). The CMS is one of the primary, general-purpose particle detectors stationed along CERN’s massive underground rings in Switzerland. Researchers utilized the CMS experiment to pinpoint the unmistakable signs of quark wakes detailed in the study. The open-access findings have been published in the scientific journal Physics Letters B.
The Universe’s First Liquid
Long before stars, planets, or even atoms could form, quark-gluon plasma is widely thought to have been the very first liquid in existence. It also holds the distinction of being the hottest liquid ever known to science, reaching temperatures of several trillion degrees Celsius during its ephemeral lifetime.
Over decades of research, scientists have increasingly described QGP as a near-"perfect" liquid. In this deeply unusual state, individual quarks and gluons appear to move together as an exceptionally smooth fluid exhibiting almost zero internal friction.
This modern understanding stems from a vast body of theoretical work and experimental validation. One of the most influential theoretical frameworks was developed by Krishna Rajagopal, the William A. M. Burden Professor of Physics at MIT, alongside his academic collaborators. Known widely as the hybrid model, this predictive framework asserted that quark-gluon plasma should react dynamically like a fluid when high-energy particles travel through it.
According to the model’s hydrodynamic calculations, a fast-moving jet of quarks ought to violently disturb the surrounding plasma, leaving a distinct wake in its path and forcing the ultra-dense material to ripple and splash.
For years, experimental physicists have hunted for tangible evidence of these wakes at the Large Hadron Collider and other high-energy particle accelerators across the globe. In these monumental experiments, heavy ions—typically lead atoms stripped of their electrons—are accelerated to nearly the speed of light before being smashed directly into one another. These cataclysmic impacts briefly recreate microscopic droplets of primordial matter that survive for less than a quadrillionth of a second.
To learn anything from these fleeting events, researchers must effectively capture a high-speed snapshot of that infinitesimal moment. They then analyze the resulting complex patterns of particles to reconstruct the physical properties of the quark-gluon plasma from which they emerged.
Why Quark Wakes Were Difficult to See
Despite decades of intense study, spotting these elusive quark wakes proved remarkably difficult. Previous searches often focused on tracking pairs consisting of a quark and an "antiquark"—subatomic counterparts whose properties share the same magnitude but carry opposite signs.
When a high-energy quark moves rapidly through the plasma, a matching antiquark may be simultaneously produced, traveling at the exact same speed in the opposite direction. Historically, scientists searched for these correlated quark-antiquark pairs, expecting both particles to leave detectable wakes in the surrounding fluid.
That intuitive approach, however, created a major experimental hurdle.
"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.
Recognizing this fundamental limitation, Lee and his research team realized 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 nearby.
"It’s a matter of clarity," Lee notes, highlighting that the team had to rethink how to isolate the signal from the surrounding background noise of the collision. "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."
Using Z Bosons as a Wake Tag
Instead of searching for traditional quark and antiquark pairs in the aftermath of lead-ion collisions, the researchers adjusted their strategy. They focused on rare events in which a single quark traveled through the dense plasma in nearly the opposite direction from an elementary particle known as a "Z boson."
A Z boson is a neutral elementary particle that mediates the weak nuclear force. Crucially, it interacts very little with the surrounding quark-gluon plasma, allowing it to serve as a clean, undisturbed reference point. Furthermore, Z bosons are produced at a highly distinctive energy level, making them relatively easy for physicists to identify amidst the debris of high-energy 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 specific collision occurs, the newly created quark and Z boson fly away from each other in nearly opposite directions. While the quark plunges into the surrounding plasma and disturbs it—leaving a physical wake in its path—the Z boson passes straight through the material without significantly affecting the fluid around it.
This mechanical asymmetry means that any ripples, splashes, or hydrodynamic patterns appearing on the quark’s side of the collision can be confidently attributed to the quark itself.
Collaborating closely with Professor Yi Chen’s research group at Vanderbilt University, the MIT team realized they could leverage Z bosons as a reliable "tag" to locate and measure the wakes created by individual, isolated quarks.
Wakes Found Among Billions of Collisions
Armed with this innovative technique, the team dove into vast archives of data gathered from heavy-ion collisions at the Large Hadron Collider. Sifting through an astounding 13 billion individual collisions, the researchers successfully identified roughly 2,000 specific events in which a high-energy Z boson was produced alongside a recoiling quark.
For each of these targeted events, the researchers meticulously mapped how energy was distributed throughout the short-lived quark-gluon plasma. Across multiple data sets, they repeatedly found clear, fluid-like patterns of splashes and swirling motion localized precisely in the direction opposite the Z boson.
Because the accompanying Z boson barely interacts with the plasma, the researchers could definitively attribute these distinct wake patterns directly to individual quarks plowing through the material.
Remarkably, the observed wakes matched theoretical predictions generated by Rajagopal’s hybrid model with striking accuracy. The results provide powerful empirical confirmation that quark-gluon plasma truly does respond collectively like a liquid when energetic particles force their way 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 in the current 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 pioneering research was supported, in part, by the U.S. Department of Energy.
