Equilibrated fraction of QCD matter in high-energy oxygen--oxygen collisions

Abstract

We quantify to what degree the QCD matter created in high-energy oxygen--oxygen ($\mathrm{O}+\mathrm{O}$) collisions at $\sqrt{s_{NN}} = 5.36$ TeV reaches a locally equilibrated state. For this purpose, we employ a novel framework based on the core--corona picture that describes the dynamics of both locally equilibrated fluids (the core) and nonequilibrium particles (the corona). Contributions from the core become larger than those from the corona above charged-particle multiplicity at midrapidity, $\langle dN_{\mathrm{ch}}/d\eta\rangle_{|\eta|<0.5} \approx 20$. We also find that nonnegligible contributions from the corona still remain even in central $\mathrm{O}+\mathrm{O}$ collisions. The yield ratios of strange baryons to charged pions exhibit an increasing behavior with increasing multiplicity at midrapidity. However, these ratios are smaller than those obtained when assuming that QCD matter has reached complete chemical equilibrium. These results demonstrate that a purely hydrodynamic approach is insufficient and that the inclusion of a corona component is essential for describing the dynamics of intermediate-size systems such as $\mathrm{O}+\mathrm{O}$ collisions.