Oxygen–oxygen collisions at the LHC show only partial local equilibration: a fluid core grows above ~20 charged particles but a nonequilibrium corona remains
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.