CERN Confirms Quark-Gluon Plasma Behaves as Coherent Fluid in Lead Ion Collisions

Scientists at the European Organization for Nuclear Research (CERN) have provided definitive confirmation regarding the fundamental nature of the quark-gluon plasma (QGP), the primordial 'soup' that dominated the universe moments after the Big Bang. The core finding establishes that the QGP, historically theorized to exist only for a fraction of a second, consistently exhibits the behavior of a highly coherent and dense fluid.

This key experimentation took place at the Large Hadron Collider (LHC), the 27-kilometer accelerator located near Geneva, Switzerland, which is designed to replicate the extreme conditions of the early universe. To achieve this primordial state, researchers collided heavy lead ions at speeds approaching the speed of light within the LHC. These high-energy collisions cause matter to melt into QGP, a phase where quarks and gluons are liberated from the confinement of hadrons such as protons and neutrons.

The research team, led by Professor Yen-Jie Lee of the Massachusetts Institute of Technology (MIT), observed measurable traces, or 'wakes,' left by high-energy quarks as they traversed the QGP medium. This phenomenon is analogous to the waves left by a boat moving across water, strongly supporting the view that the QGP acts as a true fluid rather than a randomly dispersed gas of particles. Professor Lee stated that the plasma is dense enough to slow the quarks and generate fluid-like 'splashes and vortices,' confirming its status as the primordial soup.

The analysis employed a highly specific technique: searching for rare events where high-energy quarks were produced simultaneously with the neutral Z boson. The Z boson serves as a 'silent' marker because it is virtually unaffected by the QGP, allowing scientists to accurately determine the trajectory of the quarks passing through the medium. This sophisticated methodology, utilized by the CMS (Compact Muon Solenoid) team, analyzed approximately 2,000 'golden' events from 13 billion lead-lead (PbPb) collisions collected at an energy of 5.02 TeV per nucleon in 2018.

By comparing the distribution of low-momentum particles (low pT) relative to the Z boson's direction, researchers identified asymmetries consistent with hydrodynamic wakes—the depletion and accumulation of particle density at specific angles. These results align with predictions from theoretical models, including hybrid models developed by MIT physicist Krishna Rajagopal and his collaborators, which specifically forecast fluid wakes caused by the jet-like dragging of the medium by quarks. The confirmation that QGP is a nearly perfect fluid—characterized by extremely low internal viscosity—carries profound implications for cosmology, as its collective dynamics may have influenced energy and density redistribution as the universe cooled and transitioned into the hadronic phase.

Researchers plan to apply this Z boson-based analysis technique to larger datasets to measure plasma properties in greater detail, such as the spread and dissipation rate of the wakes. Further discussion on QGP properties, including heavy quark probes and anomalous production, is scheduled for upcoming conferences, such as SQM2026 at the University of California Los Angeles (UCLA) from March 22–27, 2026.