Oxygen collisions reveal how quark–gluon matter approaches equilibrium
High-energy nuclear collisions recreate extreme conditions similar to those in the early universe, producing quark–gluon plasma (QGP), a hot state of matter in which quarks and gluons are no longer confined inside protons and neutrons. While large systems such as lead–lead collisions can produce matter that behaves like a fluid, collective behavior has also been observed in smaller systems.
In high-energy oxygen–oxygen (O + O) collisions at CERN's Large Hadron Collider, researchers have quantified how closely produced quark–gluon matter approaches thermal equilibrium. Utilizing the dynamical core–corona initialization (DCCI2) model, a team led by Professor Tetsufumi Hirano from Sophia University in Tokyo evaluated the matter produced in these collisions.
They found that when the charged-particle multiplicity exceeds around 20, the equilibrated core component begins to dominate over the nonequilibrated corona component. However, even in the most central collisions, the corona still accounts for about 30% of the total hadron yield. This indicates that while the matter exhibits fluid-like behavior in larger systems, oxygen collisions represent an intermediate regime.
Strange-baryon production ratios also remained below expectations for complete chemical equilibrium, suggesting that a nonequilibrated component persists. The findings underscore the limitations of using relativistic hydrodynamics alone to describe O + O collisions and provide a quantitative baseline for interpreting future LHC and RHIC measurements, as well as investigating how QGP formation varies with system size.
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