Sophia University quantifies quark-gluon
Researchers from Sophia University have quantified how close quark-gluon matter comes to thermal equilibrium in oxygen-oxygen collisions at the LHC.
A research team from Sophia University has quantitatively evaluated, for the first time, the extent to which quark-gluon matter produced in high-energy oxygen-oxygen collisions reaches thermal equilibrium. The work, led by Professor Tetsufumi Hirano with graduate student Naoya Ito, was published in the journal Physical Review C on August 27, 2026.
High-energy nuclear collisions at facilities like CERN's Large Hadron Collider recreate conditions of the early universe, producing a hot state of deconfined quarks and gluons known as quark-gluon plasma. While large collision systems like lead-lead produce fluid-like behavior, the status of smaller systems like oxygen-oxygen collisions has been unclear. The Sophia University team applied their dynamical core-corona initialization model to oxygen-oxygen collisions at 5.36 TeV to separate equilibrated matter from non-equilibrated particles.
Separating core from corona
The DCCI2 model separates the collision output into two components. The 'core' consists of matter that reaches local thermal equilibrium and can be described by relativistic hydrodynamics. The 'corona' represents particles that do not fully equilibrate. This approach allowed the researchers to measure collective behavior without assuming the entire system is in equilibrium. They analyzed how the balance between core and corona changed with the charged-particle multiplicity produced near the collision center.
A threshold for fluid behavior
The analysis revealed a clear transition. When the charged-particle multiplicity at midrapidity exceeded approximately 20, the contribution from the equilibrated core became larger than that from the corona. However, the corona component did not vanish. Even in the most central oxygen-oxygen collisions, it accounted for about 30% of the total hadron yield.
Professor Hirano stated the team was able to quantitatively clarify how far the produced matter approaches thermal equilibrium. The result places oxygen-oxygen collisions in an intermediate regime between systems dominated by non-equilibrated particles and those where fluid-like behavior is dominant.
Further momentum analysis showed the core contribution generally dominates at lower momenta, while the corona becomes more important at higher momenta. This transition occurs at higher momentum for heavier particles, which receive a stronger boost from the collective expansion of the core.
The team also examined strange-baryon production. Ratios of strange baryons to charged pions increased with collision multiplicity but remained below values expected for complete chemical equilibrium. This pattern provides additional evidence that a non-equilibrated component persists.
Limits of the fluid picture
"Understanding the degree of equilibration is essential because studies of oxygen nuclei and their possible alpha-cluster structure rely on the assumption that the produced matter behaves like a fluid," Hirano explained. The findings show that relativistic hydrodynamics alone cannot fully describe these collisions due to the substantial non-equilibrated component. This work provides a quantitative baseline for interpreting future LHC measurements and for investigating how quark-gluon plasma formation changes with system size. The researchers plan to extend their framework to other intermediate systems, such as neon-neon collisions.
