In the first microseconds after the Big Bang, the universe was not made of atoms or even protons and neutrons. It was a searing soup of quarks and gluons, the fundamental building blocks of matter, so hot and dense that particles could not exist as individuals. That soup, called the quark-gluon plasma (QGP), vanished within a millionth of a second as the universe expanded and cooled. To study it today, physicists must recreate it: by smashing heavy lead nuclei together at nearly the speed of light.
Now, after a 20-year search, the CMS experiment at CERN’s Large Hadron Collider has confirmed a phenomenon predicted in 2004, a “diffusion wake” left behind when an energetic particle punches through the quark-gluon plasma. The observation, accepted for publication in Physical Review Letters, provides a new window into the properties of the primordial soup that filled the early universe.
What the Wake Is
Think of the QGP as a fluid so hot and dense that even quarks and gluons, normally confined inside protons and neutrons, roam freely. When a high-energy quark or gluon (a “parton”) is produced inside this plasma, it tears through the medium, dumping energy and momentum as it travels. This creates a disturbance analogous to the wake behind a boat: a compression wave ahead of the particle and a region of depleted material behind it. That depletion, a measurable deficit of low-energy particles in the direction opposite the jet, is the diffusion wake.
“The diffusion wake had been predicted by theory over 20 years ago, but remained elusive in the experimental data,” said Prof. Olga Evdokimov of the University of Illinois Chicago, who led the analysis along with postdoc Raghunath Pradhan.
A Question of Statistics
The difficulty in finding the wake was not about whether it existed, theory said it must, but about whether the signal could be extracted from an overwhelming background of noise. Each lead-lead collision at the LHC produces thousands of particles. Among them, the wake’s signal is tiny: a subtle deficit of charged particles with transverse momentum between 1 and 2 GeV, roughly a thousandth of the energy of the jets themselves.
Previous attempts by the ATLAS experiment at the LHC, using photon-jet events, found nothing, only upper limits on what could exist. The CMS team succeeded by using a different approach: dijet events, where two back-to-back jets are produced from a single hard scattering. Dijet events occur far more frequently than photon-jet events, giving the statistical power needed to extract the signal.
The team compared dijet-hadron correlations in lead-lead (PbPb) and proton-proton (pp) collisions, both recorded at a center-of-mass energy of 5.02 TeV. By selecting events where the two jets were widely separated along the detector’s longitudinal axis, and subtracting the effects from events where the jets were close together, the diffusion wake signal emerged. It reached a statistical significance greater than 5 standard deviations, the gold standard for a discovery in particle physics.
A Window Into the Early Universe
The QGP exists for only about 10⁻²³ seconds per collision, but studying its properties tells physicists how matter behaved in the first moments after the Big Bang. “Observing and quantifying the QGP diffusion wake opens the door to new precision characterization of the properties and dynamics of the quark-gluon plasma,” Pradhan said, “and promises new insights into the evolution of the early universe.”
The wake signal was strongest in the most central collisions, where the QGP is largest and hottest, and disappeared in peripheral collisions where less plasma is produced. The signal was clearest for charged particles in the 1-2 GeV range, with a smaller but still significant signal seen between 2 and 4 GeV.
Caveats and Tensions
The existing theoretical models that include wake effects, the HYBRID model and the CoLBT-hydro model, both predict the observed depletion, but they overestimate its magnitude. This means the theoretical understanding of how the QGP responds to energetic jets is still incomplete. No model perfectly matches the data, leaving room for refinement.
A previous ATLAS search for the same phenomenon using photon-jet events (Phys. Rev. C 111, 044909, 2025) found only upper limits. The CMS team’s success with the dijet method suggests the photon-jet approach was simply not sensitive enough, dijet events provide significantly more statistical power.
What’s Next
With LHC Run 3 ending on June 29, 2026, the CMS detector now enters its Long Shutdown 3, during which the LHC will be upgraded for Run 4, expected to begin in 2030. The new data from higher luminosity collisions will allow even more precise measurements of the wake and its properties. Future work may also probe the wake at different energies and with different collision systems, potentially revealing new aspects of QGP behavior.
The paper, titled “Observation of the jet diffusion wake using dijets in heavy-ion collisions,” is authored by the CMS Collaboration (A. Hayrapetyan et al.) and accepted for publication in Physical Review Letters. The preprint is available on arXiv: 2602.19431 (nucl-ex).
Sources
1. CMS Collaboration, “Observation of the jet diffusion wake using dijets in heavy-ion collisions,” accepted by Physical Review Letters, arXiv:2602.19431 (2026). DOI: 10.1103/g49y-8cjl
2. CMS Experiment news article, “In the wake of partons,” July 8, 2026. cms.cern/news/wake-partons
3. R. Lea, “Earth’s largest particle accelerator opens new window into the early universe,” Space.com, July 17, 2026.