CERN Scientists Observe Quark Wake in Primordial Plasma, Revealing How the Universe First Flowed
Just after the Big Bang, the newborn universe was an intensely hot sea of quarks and gluons, heated to-trillions of degrees. These particles shot around at near light speed, forming a fleeting substance called Quark-Gluon Plasma (QGP) that existed for only millionths of a second.
As temperatures fell, the plasma cooled and condensed, giving rise to protons, neutrons and the fundamental matter that makes up the universe today.
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Recreating the Universe's First Moments at CERN
Scientists at CERN's Large Hadron Collider are now recreating this early cosmic state to better understand how the universe began. By colliding heavy ions enormous energies, they can momentarily recreate quark-gluon plasma and study matter as it existed in the universe's first instants.
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Breakthrough Discovery of Quark Wake Effects
In a major breakthrough, a CERN research team led by MIT physicists has detected clear signs that fast-moving quarks leave wake-like trails in the plasma.
Much like water rippling behind a moving bird, the plasma responds as a single fluid—offering the first direct evidence that it flows and splashes rather than behaving like a collection of independent particles.
Quark-Gluon Plasma Behaves Like a Primordial Fluid
"There has been a long-running question in our field about whether the plasma reacts to a quark moving through it," explained MIT physicists Yen-Jie Lee.
"Our results show the plasma is so dense that it can actually slow a quark down, producing ripples and swirls like a fluid. Quark-gluon plasma truly behaves like a primordial soup."
A New Technique to Detect Quark-Induced Wakes
To observe these wake-like signatures. Lee and his colleagues devised a new experimental technique, which they describe in their latest paper. The team intends to use the approach on further collision data to uncover more examples of quark-induced wakes.
Tracking how large these wakes are, how fast they spread and how quickly they fade allows scientists to probe the nature of the plasma and reconstruct how it may have flowed in the universe's first fleeting moments.
"By examining how quark wakes ripple and rebound, we can uncover fresh insights into the properties of quark-gluon plasma," said Lee. "This experiment effectively captures a snapshot of that primordial quark soup."
Quark Shadows and Theoretical Predictions
The study was co-authored by members of the CMS Collaboration, an international network of particle physicists who jointly operate and analyze data from the Compact Muon Solenoid (CMS), one of the Large Hadron Collider's main detectors at CERN.
The CMS experiment played a crucial role in identifying the wake effects created by quarks.
The research has been published as an open-access paper in the journal Physics Letters B.
Quark-Gluon Plasma — The Universe's First Liquid
The Hottest Matter Ever Observed
Quark-gluon plasma is believed to be the very first liquid to form in the universe—and the hottest ever observed. During its fleeting existence, scientists estimate its temperature reached several trillion degrees Celsius.
This seething state of matter is also thought to have behaved as a near-"perfect" liquid, with quarks and gluons flowing together smoothly, almost without friction.
Theoretical Models Predicting Plasma Wakes
This understanding is supported by a wide range of experiments and theoretical studies. One influential model, developed by MIT physicist Krishan Rajagopal and his colleagues, predicts that the plasma should behave like a fluid when disturbed by fast-moving particles.
Known as the hybrid model, the theory suggests that a jet of quarks racing through the plasma would generate a wake, causing the quark-gluon plasma (QGP) to ripple and splash.
The Challenge of Detecting Quark Wakes
Physicists have spent years searching for these wake effects in experiments at the Large Hadron Collider and other high-energy accelerators.
In these tests:
- Heavy ions such as lead are accelerated to near light speed
- Collisions briefly create tiny droplets of quark-gluon plasma
- The plasma survives for less than a quadrillionth of a second
Researchers effectively capture a snapshot of this fleeting moment to uncover the plasma's defining properties.
Why Quark-Antiquark Pairs Complicate Detection
To spot quark wakes, scientists have focused on pairs of quarks and antiquarks— particles that mirror quarks but carry opposite properties.
When a quark races through the plasma, its matching antiquark is expected to move at the same speed in the opposite direction.
"When two quarks are produced, the challenge is that, as they fly off in opposite directions, one quark can mask the wake created by the other," said Lee.
A Wake Tag Using Z Bosons
Isolating a Single Quark's Wake
Instead of searching for quark-antiquark pairs, Lee's team focused on rare events in which a single quark travels through the plasma in the opposite direction to A Z boson.
A Z boson is electrically neutral and interacts only weakly with its surroundings, leaving the plasma largely undisturbed.
Why Z Bosons Are Ideal Markers
"In this quark-gluon soup, countless quarks and gluons are constantly colliding," Lee explained.
Occasionally, one collision produces:
- A high-momentum quark
- A Z boson recoiling in the opposite direction
Any ripples observed in the plasma are therefore created solely by the single quark.
Analyzing Billions of Collisions
Working with Professor Yi Chen's team at Vanderbilt University, the researchers used Z bosons as a reliable "tag" to trace quark wakes.
From around 13 billion heavy-ion collisions, they identified roughly 2,000 events that produced a Z boson.
By mapping energy patterns, they repeatedly observed:
- Fluid-like splashes
- Swirling wake structures
- Clear signatures of single quarks slicing through the plasma
Clear Proof of Fluid Behaviour
The wake patterns closely matched predictions from Rajagopal's hybrid model, confirming that quark-gluon plasma truly behaves like a fluid.
"This is something many of us have argued must exist for a long time," said Rajagopal.
"What the CMS team have achieved is a measurement that finally delivers the first clean, clear and unambiguous evidence of this fundamental effect," added Daniel Pablos of the University of Oviedo.
A New Window Into the Early Universe
"This is the first clear, direct proof that a travelling quark drags surrounding plasma in its wake," Lee said.
"The findings opens the door to studying the nature and dynamics of this extraordinary fluid with unprecedented precision."
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