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Researchers studied how much of the matter produced in high-energy oxygen–oxygen collisions at the Large Hadron Collider (LHC) behaves like a locally equilibrated fluid. They used a two-part picture called the core–corona model. The “core” is matter that has thermalized and can be treated like a fluid. The “corona” is particles that remain far from equilibrium. The main result is that the fluid-like core produces more hadrons than the corona once the charged-particle multiplicity at midrapidity passes about 20, but the corona still contributes noticeably even in the most central collisions.
To reach these conclusions the authors ran a dynamical simulation called DCCI2 (Dynamical Core–Corona Initialization, version 2). The simulation starts from many initial partons created by the PYTHIA Angantyr event generator. Partons that are in dense neighborhoods deposit energy and momentum into a fluid described by three-dimensional relativistic hydrodynamics. Partons that do not thermalize continue on as nonequilibrium “corona” particles and hadronize using string fragmentation. When the fluid cools below a switching temperature of 165 MeV it is converted into hadrons with a Monte Carlo Cooper–Frye procedure. The final hadrons are passed through decay routines, and the study turned off later-stage hadronic scatterings so that the relative core and corona contributions could be separated cleanly. The authors generated 100,000 minimum-bias events for the analysis.
This question matters because oxygen nuclei are intermediate in size between protons and heavy nuclei such as lead. That makes oxygen–oxygen collisions a useful test case for when and how the quark–gluon plasma (QGP) — the hot, strongly interacting matter governed by quantum chromodynamics (QCD) — forms a locally equilibrated fluid. If local equilibrium is reached, hydrodynamics can reliably connect the collision geometry to the momentum patterns of produced particles. If equilibrium is only partial, a hydrodynamic-only description will miss important nonequilibrium contributions.
The authors also looked at particle composition. Ratios of strange baryons to charged pions rise with increasing multiplicity at midrapidity, which is consistent with a growing core fraction. However, these ratios remain smaller than what would be expected if the entire system had reached full chemical equilibrium. The paper reports that a purely hydrodynamic model that assumes complete equilibration is therefore insufficient to describe intermediate-size systems like oxygen–oxygen collisions.
There are important caveats. The results depend on model choices: the treatment of initial partons, the parameters used in PYTHIA (for example pT0Ref = 1.0 GeV), the assumption of ideal hydrodynamics, and the switching temperature for hadronization. The study disabled hadronic rescatterings to isolate core and corona contributions; the authors say rescatterings have only a small effect on final yields, but including them mixes the two components. Finally, these findings come from one simulation framework and from the excerpted text; further cross checks with other models and data will be needed to firm up the conclusions.