Quarks leave a “trace” in the primordial oceans of the universe: CMS and MIT bring the clearest evidence that quark-gluon plasma behaves like a liquid
In the first microseconds after the Big Bang, the universe was not filled with atoms, stars, or even protons and neutrons, but with an extremely hot and dense mixture of quarks and gluons. This phase, known as quark-gluon plasma (QGP), is considered the earliest form of matter: it existed briefly as the universe cooled, and then the quarks and gluons “locked” into hadrons, the building blocks of today's matter. This is precisely why QGP is one of the few links between fundamental particle physics and cosmology – the same laws can be tested in the laboratory and then incorporated into the broader picture of how the structure of the universe developed from its initial state.
Lead ion collisions as a window into the earliest universe
To understand this initial “recipe,” physicists at CERN’s Large Hadron Collider (LHC) collide heavy ions – most often lead nuclei – at speeds close to the speed of light. In these collisions, for a fraction of a fraction of a second, a droplet of QGP is created with temperatures measured in trillions of degrees. Although it lasts for an extremely short time, traces of its behavior remain recorded in the distribution of particles that fly out of the collision and are registered by detectors like CMS (Compact Muon Solenoid). Analyses rely on the fact that the statistics of a large number of collisions, combined with precise reconstruction of trajectories and energies, allow a stable pattern – a kind of “photograph” of the most exotic state of matter we can produce today – to be extracted from the noise of events.
Long-standing debate: particle scattering or collective flow?
One of the key questions in the physics of strong interactions was how QGP reacts when a “hard,” high-energy particle, such as a quark produced in a collision, flies through it. If QGP behaves as a set of loosely connected particles, the passage of a quark should look like a series of random collisions and scatterings, without an orderly pattern in the “soft,” low-momentum particles. But if QGP is indeed a collective, nearly “perfect” fluid with very low viscosity, then the passage of a quark should trigger a response similar to a wave or a wake: the energy the quark loses pulls the surrounding medium and turns into an organized flow and density perturbations that spread like waves in water.
Precisely this hydrodynamic wake is what physicists have been trying to capture in the data for years, but the signal was often lost in the background of other processes. A key problem was that quarks are typically created in pairs with an antiparticle: when one jet goes in one direction, the other goes in the opposite direction, and their traces in a complex collision can overlap. To get a “clean” case of a single jet and a single clear passage through the medium, it was necessary to find an event in which the direction of the quark could be tagged without creating a “partner” that would overshadow the image.
Why the Z boson is key: “neutral tag” without impact on the medium
An international team within the CMS collaboration, in which physicists from MIT play an important role, focused on a rare but very clean signature: events in which a Z boson and a high-energy quark are produced in the same collision. The Z boson is a neutral particle of the weak interaction that passes through the QGP practically undisturbed, without leaving its own “wave.” This makes it an ideal “tag” that shows where the high-energy quark jet was created in the event, while simultaneously not blurring the image of the medium.
The logic is simple: the Z boson and the quark are produced “back-to-back,” in opposite directions. The direction of the Z boson gives a precise vector according to which the coordinate system of the analysis can be defined. Everything that happens in the QGP on the opposite side, where the quark and its jet pass, can be largely attributed specifically to the interaction of the quark with the medium. In practice, this means looking at the distributions of charge and energy of low transverse momentum particles relative to the angle and pseudorapidity toward the Z boson, and comparing them with reference situations where QGP is not present.
From 13 billion collisions to about 2000 “golden” events
In the analysis of data collected in heavy-ion collisions, the CMS team searched through about 13 billion events and identified approximately 2000 cases with a Z boson that meets strict criteria of high transverse momentum, in the range of 40 to 350 GeV. The analysis relied on PbPb collision data collected in 2018 at a collision energy per nucleon pair of 5.02 TeV, with an integrated luminosity of about 1.67 nb⁻¹, and on comparative pp data from 2017 at the same energy (about 301 pb⁻¹). After selecting the events, researchers “translated” each collision into a map of the distribution of low pT particles relative to the direction of the Z boson, looking for asymmetries that would be the signature of the medium's response to the passage of the jet.
The result is a pattern that physicists describe as a consistent modification of distributions on the side opposite the Z boson: simultaneous enhancements and depletions appear in certain angular regions, corresponding to the image of the medium being “pulled” and creating areas of particle deficit (hole) and excess (recoil). In CMS's preliminary results published on October 9, 2024, it was highlighted that in PbPb collisions, a significant change in azimuthal and pseudorapidity distributions is seen relative to the pp reference, specifically in the low pT region around 1–2 GeV. In the final, peer-reviewed version published as a letter in the journal Physics Letters B, the emphasis was placed on interpretation: the observed patterns are consistent with a hydrodynamic “wake” that occurs when a jet depletes energy from the quark-gluon plasma, and the medium then responds collectively.
What a “wave” in quark-gluon plasma actually means
In a classic liquid, a wave occurs because equilibrium is disturbed: a moving body pushes the surrounding medium, creating areas of excess and deficit density, and then the disturbance spreads and dampens. In QGP, the image is quantum and relativistic, but the analogy is useful: a quark or jet loses energy and momentum in the medium, and the QGP responds to this with collective flow and redistribution of energy into “soft,” low-momentum particles. It is precisely in this soft sector – which is traditionally difficult to link to a specific “hard” particle – that proof is sought that the plasma does not react randomly, but as a whole.
In the CMS paper itself, it is emphasized that the observed correlations are in line with the expectations of a hydrodynamic wake: when a jet “extracts” energy from the plasma, a depletion area remains behind it, while part of the medium accelerates and “pushes away” in other directions, creating an excess of low pT particles at characteristic angles. In practice, this looks like a complex pattern of “splashes” and “vortices” in the distributions, but the key message is simple: the reaction is not random, but collective and fluid. In other words, QGP does not behave like a dilute cloud of particles, but like a dense state in which disturbances spread through the medium and leave a recognizable imprint.
Hybrid models and why data matches theory
The significance of the result is not only that the signal was observed, but also that it can be directly related to theoretical predictions. In descriptions of QGP today, two worlds are often combined: a quantum description of jets (parton showers) created in a hard collision and a hydrodynamic description of the medium behaving like a liquid. Such “hybrid” approaches allow the energy lost from the jet not to be treated as gone, but as something that ends up in the collective flow of the QGP. In the arXiv version of the paper and the published letter, it is emphasized that the observed modifications are compatible with the expectations of precisely such models, which predict the simultaneous existence of a “hole” in the direction of the jet and a “response” of the medium through the enhancement of soft particles.
Physicists caution that this is the first evidence in this specific measurement channel: correlations of the Z boson with soft hadrons in heavy-ion collisions. The strength of the method is that the Z boson acts as a “silent witness” to the event, so the direction and energy of the jet that passed through the plasma can be more accurately reconstructed from it, without additional signal contamination. Such an approach reduces the ambiguities that plagued earlier attempts to pair two opposite jets, where it was difficult to separate “who overshadowed whose trace.”
Why this matters for the broader picture of the early universe
While QGP has long been described as a medium that behaves like a nearly “perfect liquid,” proving a collective response to a single high-energy particle carries additional weight. Such measurements provide access to the transport properties of the plasma: how efficiently it transfers energy and momentum, how quickly a disturbance is dampened, and how energy is distributed in space and time. These are parameters behind concepts like viscosity and diffusion, and in the extreme conditions of QGP, they are key to understanding how “hard” processes (jet formation) intertwine with the “soft” collective behavior of the medium.
In a broader sense, this is also a test of the early universe's image. According to CERN’s overviews of the history and heavy-ion program, quark-gluon plasma dominated the universe less than ten microseconds after the Big Bang. During this period, matter was so hot and dense that quarks and gluons could not be bound into protons and neutrons. If the QGP indeed behaved as a liquid then, its collective dynamics could have influenced the way energy and density were redistributed as the universe cooled and transitioned into the hadronic phase. Today's measurements do not “reconstruct” the cosmological timeline directly, but they directly test the physics that must have been active in it.
What follows: more precise “mapping” of the wake in the plasma
The authors emphasize that the Z boson method opens the door to more systematic analyses. By increasing the sample size and analyzing different classes of collision centrality, it is possible to more accurately determine the geometry and “thickness” of the medium through which the jet passes. Comparison with simulations will allow an estimation of how quickly the wake spreads, how long it persists before “leveling out” in the particle noise, and how it depends on the energy and type of the initial parton. On the practical side, this is a path toward more precise determination of QGP properties in a way that is robust to experimental and theoretical uncertainties, as it relies on a particle (the Z boson) that is extremely well “calibrated” and poorly sensitive to the medium.
For the audience outside the narrow circle of experts, the main message is clear: the laboratory is increasingly showing that the “primordial soup” of the universe was indeed a soup – a medium that flows and reacts as a whole. When a quark speeds through that droplet created in a lead collision, it can leave a trace behind that is read like a wave on water, only on a scale where temperatures, densities, and speeds are beyond everyday experience, and conclusions are drawn from precise statistical patterns of billions of collisions.
Sources:- Physics Letters B – open access letter from the CMS collaboration on Z boson-hadron correlations and evidence of probe-induced energy and medium response (link)
- arXiv – preprint CMS-HIN-23-006 (arXiv:2507.09307) describing measurement and interpretation of the hydrodynamic wake signal (link)
- CMS Public Results – preliminary result HIN-23-006 (October 9, 2024) and summary of observed modifications in PbPb collisions (link)
- CERN – “CERN70: Tasting the primordial soup” overview of quark-gluon plasma and the early universe (link)
- U.S. Department of Energy – “The Big Questions: Barbara Jacak on the Quark-Gluon Plasma” on the purpose and methods of creating QGP in the laboratory (link)
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