Deep within the Large Hadron Collider (LHC) – the world's most powerful particle accelerator – scientists have finally observed what has been elusive for decades. This refers to the "diffusion wake" left by a fast quark or gluon as it passes through quark-gluon plasma, a super-hot and dense "soup" of matter's fundamental constituents, resembling the state of the Universe in the first microseconds after the Big Bang.
Imagine this: two lead nuclei are accelerated to near light speed and collide within the CMS detector. At this moment, quark-gluon plasma is born – a medium where quarks and gluons (partons) exist freely, not confined within protons and neutrons. When a high-energy parton flies through this plasma, it loses energy and momentum, leaving behind a disturbance similar to the wake of a boat in water. Theory predicted such an effect over 20 years ago, but it had been difficult to reliably capture experimentally – the signal was too weak against the background of other processes.
Previously, scientists searched for traces in events with jets (streams of particles) and Z bosons, but noise from other effects masked the picture. A team led by researchers from the University of Illinois at Chicago (UIC), including Raghunath Pradhan and Olga Evdokimov, employed a new approach. They focused on di-jet events – where two jets emerge in almost opposite directions. This allowed for better separation of the wake signal from the background.
Analysis of lead-lead collision data at an energy of 5.02 TeV per nucleon showed a clear pattern: a noticeable depletion of low-momentum particles (in the range of 1–2 GeV) was observed behind the jets. The effect intensifies in central, more "dense" collisions, where more plasma is formed. The significance exceeded five standard deviations – a level considered a robust discovery in particle physics.
"This is the culmination of years of searching," noted Olga Evdokimov. "Observing and quantitatively describing the diffusion wake opens the door to precisely characterizing the properties of quark-gluon plasma and provides new insights into the evolution of the early Universe."
The results, accepted for publication in Physical Review Letters (paper HIN-25-012), do not just confirm the theory. They help us better understand how matter behaved in the very first moments of the cosmos – when the first protons, neutrons, and ultimately, the entire visible Universe formed from this plasma. The plasma behaves like an ideal liquid, interacting strongly with particles passing through it, rather than like a rarefied gas.
For the general public, this serves as a reminder of how laboratories on Earth allow us to glimpse conditions inaccessible to direct observation. Each new "wake" captured in the collider brings us closer to understanding how the ordered cosmos in which we exist emerged from the chaos of the initial moments. And the search, of course, continues.
