The Large Hadron Collider caught quarks leaving wakes in the Big Bang’s primordial soup

Yen-Jie Lee, the MIT physics professor who led the analysis, puts the result in a single line: quark-gluon plasma really is a primordial soup. His team found that a quark speeding through that plasma drags some of it along and leaves a wake behind it. The evidence sits in data recorded by the CMS detector at CERN’s Large Hadron Collider.

The plasma in question is the matter that filled the universe for its first few millionths of a second, at a temperature MIT puts at a few trillion degrees Celsius. The same account calls it the first liquid. That label matters here, because a liquid is the kind of medium that can carry a wake at all, and the question the experiment asked was whether the plasma answers a fast-moving quark as a single fluid or as a loose crowd of particles that simply scatter. Physicists at the collider in Switzerland recreate tiny droplets of it by accelerating lead ions to nearly the speed of light and smashing them together, and the CMS Collaboration then analyzes what comes out of the debris.

The result comes from the CMS Collaboration, one of the collider’s general-purpose experiments, and it was published in Physics Letters B 874 (2026), article 140120, after a preprint appeared on arXiv on July 12, 2025. MIT News, in a story by Jennifer Chu, told the story on January 28, 2026, and a ScienceDaily summary of that coverage reached readers on October 6. The paper, “Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions,” is open access, and its journal record carries the DOI 10.1016/j.physletb.2025.140120.

Z bosons as a clean tag

The difficulty with seeing a wake is that quarks produced in these collisions come in pairs, and two quarks muddy each other’s wakes. MIT’s account explains the workaround: pair a single quark with a Z boson. The Z barely interacts with the plasma, so it works as a tag, and the quark produced back-to-back with it can be studied without a second quark’s disturbance.

The statistics are lopsided by design. The team went through about 13 billion heavy-ion collisions and picked out roughly 2,000 in which a Z boson was produced. In those events the wake appeared as splashes and swirling motion on the side opposite the Z boson, a pattern consistent with a liquid responding to a moving object.

CMS lead-lead data at 5.02 TeV

The arXiv abstract frames the physics in terms of recoil: the measurement probes how the plasma affects the parton recoiling opposite a high-momentum Z boson, and the results agree with a hydrodynamic wake, meaning the plasma responds to energy lost to a probe moving through it. In plain terms, the plasma behaves as one fluid, not a gas of independent particles.

The collaboration’s own publication page gives the technical frame. It reports the first measurement of charged-hadron distributions in pseudorapidity and azimuthal angle relative to a Z boson’s momentum, in lead-lead collisions at 5.02 trillion electron volts per nucleon pair, compared against proton-proton references. The lead-lead sample comes from 2018 data at 1.67 inverse nanobarns, the proton-proton reference from 2017 data at 301 inverse picobarns, and the Z bosons span transverse momenta from 40 to 350 GeV.

A significant modification appears for charged hadrons with transverse momenta between 1 and 2 GeV. The authors describe this as the first evidence of medium-recoil and medium-hole effects produced by a hard probe, and MIT’s release goes a step further, quoting Lee that the team has gained the first direct evidence that the quark drags more plasma with it as it travels.

A prediction that held

The measured patterns matched a hybrid model developed by Krishna Rajagopal, MIT’s William A. M. Burden Professor of Physics, who was not directly involved in the study. Rajagopal said the wake is something that he and many colleagues had argued for years must be there. Professor Yi Chen’s group at Vanderbilt University collaborated on the work, and the U.S. Department of Energy supported it in part. Daniel Pablos of Oviedo University in Spain, a collaborator of Rajagopal’s who was also not involved, commented on the work for MIT.

Lee described the measurement to MIT as a snapshot of the primordial quark soup. The ScienceDaily version of the story, which the October 6 posting carries, repeats the same quotes and credits the lead team as MIT working with CERN.

The finding is deliberately narrow, and the paper’s own title says evidence, not proof. It is one set of lead-lead data with a Z boson as the tag, and the open numerical question is how much plasma a quark of a given energy pushes aside, which the 1-to-2 GeV hadron measurement only begins to size.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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