Physicists have confirmed what scientists have long theorized but never directly observed: the quark-gluon plasma that filled the infant universe behaved like a true fluid, sloshing and splashing in response to particles moving through it.
The discovery, announced January 27, 2026, by researchers at MIT and the CMS Collaboration at CERN's Large Hadron Collider, provides the first unambiguous evidence that this trillion-degree primordial matter reacted as a coherent liquid rather than a collection of randomly scattering particles.
The findings resolve a decades-long debate about the fundamental nature of the universe's earliest moments and offer a new window into conditions that existed mere microseconds after the Big Bang, when quarks and gluons roamed freely before combining to form the protons and neutrons that constitute all ordinary matter today.
Wakes in Primordial Matter
The breakthrough hinges on detecting the wake patterns left by quarks speeding through recreated quark-gluon plasma, analogous to ripples trailing behind a duck swimming across water.
These wakes represent direct proof that the plasma responds collectively as a unified fluid.
"It has been a long debate in our field, on whether the plasma should respond to a quark," said Yen-Jie Lee, professor of physics at MIT and leader of the research team.
"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".
The research team, working within the CMS Collaboration—an international partnership operating the Compact Muon Solenoid detector at CERN—developed an innovative technique to isolate the wake effects of individual quarks.
The study appears in the journal Physics Letters B.
Recreating the Universe's First Microsecond
Quark-gluon plasma existed during the first microsecond of the universe's existence, when temperatures reached several trillion degrees Celsius—approximately 220,000 times hotter than the core of the sun.
At such extreme conditions, matter could not exist in familiar forms. Instead, quarks and gluons, which are normally confined within protons and neutrons, moved freely through space in a dense, superheated state.
As the universe expanded and cooled, this primordial soup rapidly transformed. Quarks combined in groups of three to form protons and neutrons, which eventually assembled into atomic nuclei—the building blocks of all matter in the universe.
At CERN's Large Hadron Collider, physicists recreate this ancient state of matter by accelerating lead nuclei to nearly the speed of light and smashing them together with tremendous force.
These collisions generate tiny droplets of quark-gluon plasma that exist for less than a quadrillionth of a second before cooling and hadronizing back into ordinary particles.
Scientists estimate the energy density in these collisions exceeds 15 times the threshold needed to liberate quarks and gluons from their normal confinement.
The resulting plasma achieves temperatures around a few trillion degrees Celsius, briefly recreating the conditions of the newborn universe in a space smaller than an atomic nucleus.
The Challenge of Detecting Single-Quark Wakes
Previous attempts to observe wake effects faced a fundamental obstacle. When physicists looked for quark-antiquark pairs produced in heavy-ion collisions, the particles flew off in opposite directions.
The wake from one quark would obscure the wake from the other, making it impossible to definitively attribute the observed patterns to fluid-like behavior.
"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 explained.
The MIT-led team devised a solution by searching for rare collision events that produce a Z boson alongside a single quark. Z bosons—electrically neutral elementary particles associated with the weak nuclear force—have virtually no interaction with the surrounding quark-gluon plasma.
When detected, they serve as a reference marker indicating the precise opposite direction of the quark, without creating their own wake effects.
"In this soup of quark-gluon plasma, there are numerous quarks and gluons passing by and colliding with each other," Lee said. "Sometimes, when we are lucky, one of these collisions creates a Z boson and a quark, with high momentum".
The team, collaborating with Professor Yi Chen's group at Vanderbilt University, used the Z boson as a "tag" to trace single-quark wake effects.
Chen, who previously worked as a CERN research fellow and led the CMS heavy-ion physics analysis group, specializes in using jets and hard probes to study quark-gluon plasma properties.
Mining Collision Data
The researchers analyzed data from 13 billion lead-lead collision events recorded at the Large Hadron Collider during heavy-ion runs. From this massive dataset, they identified approximately 2,000 events that produced a Z boson.
For each event, the team mapped the distribution of energy throughout the short-lived quark-gluon plasma droplet.
Consistently, they observed fluid-like patterns—splashes and swirls characteristic of wake effects—in the direction opposite to the Z bosons. These patterns could be directly attributed to single quarks zooming through the plasma.
The observed wake effects matched predictions from theoretical models developed by Krishna Rajagopal, the William A. M. Burden Professor of Physics at MIT, and his collaborators.
Rajagopal's hybrid model posits that when jets of quarks pass through quark-gluon plasma, they should induce the medium to ripple and splash in response, much as a solid object creates wakes when moving through water.
"This is something that many of us have argued must be there for a good many years, and that many experiments have looked for," Rajagopal said.
"What Yen-Jie and CMS have done is to devise and execute a measurement that has brought them and us the first clean, clear, unambiguous, evidence for this foundational phenomenon".
Daniel Pablos, professor of physics at Oviedo University in Spain and a collaborator of Rajagopal who was not directly involved in the current study, emphasized the significance of the breakthrough: "Yen-Jie and CMS have done is to devise and execute a measurement that has brought them and us the first clean, clear, unambiguous, evidence for this foundational phenomenon".
A Perfect Fluid
The findings confirm that quark-gluon plasma behaves as what physicists call a "near-perfect fluid"—a substance that flows with extraordinarily low viscosity, exhibiting minimal internal friction.
This contradicts earlier expectations that the plasma would behave more like a gas, with individual particles moving independently.
The plasma's viscosity-to-entropy ratio approaches the theoretical minimum allowed by quantum mechanics, making it one of the most perfect fluids ever observed.
Despite individual quarks and gluons moving at nearly the speed of light with tremendous energy, they flow together coherently as a unified medium.
Remarkably, recent calculations found that quark-gluon plasma's ratio of viscosity to density closely resembles that of water, despite being 16 orders of magnitude hotter and denser.
This unexpected similarity suggests fundamental physical constants may govern the behavior of both ordinary liquids and this exotic primordial matter.
"The viscosity of quark-gluon plasma turned out to be astonishingly close to water," researchers noted in a 2021 study published in SciPost Physics. "The two fluids will flow in the same way despite their significant differences".
Understanding the Early Universe
The ability to study quark-gluon plasma wakes opens new avenues for understanding the universe's first microsecond.
By measuring the size, speed, and extent of these wakes, and how long they take to dissipate, scientists can determine fundamental properties of the plasma itself.
"Studying how quark wakes bounce back and forth will give us new insights on the quark-gluon plasma's properties," Lee said. "With this experiment, we are taking a snapshot of this primordial quark soup".
The findings have implications beyond pure physics.
Quark-gluon plasma represents a critical phase in cosmic evolution—the transition period when the universe transformed from an undifferentiated soup of fundamental particles into the structured matter that would eventually form stars, galaxies, and planets.
Understanding how quarks and gluons behaved during this transition helps explain why the universe developed as it did.
The fact that the plasma flowed as a coherent fluid rather than a disordered gas influenced how matter began organizing itself in those crucial first moments.
Future Research Directions
The research team plans to apply their Z boson tagging technique to additional particle-collision data to study more quark wakes in greater detail.
By analyzing how wakes propagate, evolve, and dissipate under different conditions, physicists can construct increasingly precise models of quark-gluon plasma behavior.
The measurements also provide information about the "resolution length" of the plasma—the minimum separation at which the medium can distinguish individual particles within a jet shower.
Recent theoretical work by Rajagopal and colleagues suggests that the plasma has a finite, non-zero resolution length approximately 1-2 times the inverse of its temperature.
"We've gained the first direct evidence that the quark indeed drags more plasma with it as it travels," Lee said. "This will enable us to study the properties and behavior of this exotic fluid in unprecedented detail".
Heavy-ion experiments at the Large Hadron Collider continue to produce vast quantities of data during annual collision runs.
The ALICE detector, specifically designed for heavy-ion physics, and general-purpose detectors like CMS and ATLAS all contribute to understanding quark-gluon plasma properties from complementary perspectives.
These experiments recreate conditions from progressively earlier moments after the Big Bang, pushing toward understanding the very first instants when fundamental forces and particles first emerged from the primordial chaos.
The confirmed observation of quark wakes represents a crucial step in this ongoing investigation into the universe's earliest chapter.
The work received support from the U.S. Department of Energy.
