CERN Physicists Observe Quark Wakes in Primordial Plasma for the First Time Confirming the Early Universe Flowed Like a Liquid
The CMS Collaboration at CERN has detected wakes left by quarks speeding through quark-gluon plasma, providing the first direct evidence that the matter filling the universe in its first microseconds behaved as a nearly perfect liquid rather than a gas of free particles.
Overview
Physicists working on the CMS experiment at CERN’s Large Hadron Collider have observed the first direct evidence that quarks create wakes as they travel through quark-gluon plasma, the exotic state of matter that filled the universe during its first microseconds after the Big Bang. The findings, published in Physics Letters B, confirm a long-standing theoretical prediction that this primordial substance responds to fast-moving particles as a single flowing liquid rather than scattering randomly like a collection of independent particles.
What We Know
Quark-gluon plasma is the state of matter that existed when the universe was less than a millionth of a second old, before quarks and gluons cooled enough to bind into protons and neutrons. Scientists have recreated it at the Large Hadron Collider by smashing lead ions together at near-light speed, producing temperatures exceeding several trillion degrees Celsius. According to CERN’s overview of heavy-ion physics, these collisions briefly generate droplets of quark-gluon plasma that last for roughly ten sextillionths of a second before cooling and breaking apart.
The breakthrough came from a technique developed by MIT professor of physics Yen-Jie Lee and collaborators at Vanderbilt University. As reported by MIT News, the team analyzed approximately 13 billion heavy-ion collisions recorded by the CMS detector, searching for rare events that produced a Z boson alongside a quark. The Z boson is electrically neutral and passes through quark-gluon plasma without interacting with it, making it an ideal reference marker. When a quark and a Z boson are created together, they travel in opposite directions. Any disturbance observed on the quark’s side can therefore be attributed to the quark alone, rather than to the overlapping wakes of quark-antiquark pairs that had obscured previous measurements.
From the 13 billion collisions, the researchers identified roughly 2,000 events containing a Z boson. By mapping the energy distribution around each event, they observed a consistent pattern: a splash of excess energy and swirling disturbance on the side opposite the Z boson, exactly where the quark had plowed through the plasma. The pattern matched predictions from a hybrid theoretical model developed by MIT physicist Krishna Rajagopal and collaborator Daniel Pablos of the University of Oviedo, which treats quark-gluon plasma as a hydrodynamic fluid.
As described in the MIT Physics Department’s report, the observations reveal a dual wake structure: a positive wake of accumulated energy in the direction the quark travels, and a region of energy depletion behind it, similar to the bow wave and trough created by a boat moving through water.
What We Don’t Know
While the detection of quark wakes confirms the fluid-like nature of quark-gluon plasma, several properties remain to be measured precisely. The viscosity of the plasma, the speed of sound within it, and the exact mechanism by which it dissipates energy from fast-moving particles are all quantities that future analyses of the wake data may help constrain. The current dataset of roughly 2,000 Z boson events, while sufficient to establish the wake signal, limits the precision of these measurements. Future LHC runs with higher luminosity could provide significantly more events.
It also remains unclear how the properties of quark-gluon plasma measured in laboratory collisions map onto conditions in the actual early universe. The plasma droplets created at the LHC are microscopic and short-lived, while the primordial plasma filled the entire observable universe and persisted for microseconds. Whether the wake behavior observed at LHC energies holds at the even higher temperatures present in the earliest moments after the Big Bang has not been established.
Analysis
The significance of this result extends beyond confirming a theoretical prediction. The ability to measure how quark-gluon plasma responds to a known perturbation opens a new experimental avenue for studying the properties of matter under the most extreme conditions that have existed in the universe. Previous experiments had established that quark-gluon plasma behaves as a nearly perfect fluid with extraordinarily low viscosity, but those conclusions rested on indirect measurements of particle flow patterns. The wake technique provides a more direct probe, analogous to measuring the viscosity of water by dragging an object through it and observing the resulting ripples.
The study represents a collaboration across the CMS experiment’s international team, with theoretical groundwork from MIT and the University of Oviedo, and was supported in part by the U.S. Department of Energy. The paper’s title, “Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions,” reflects the technical specificity of the measurement while understating what it reveals: the stuff of the early universe really was, as the researchers describe it, soupy.