Global observables and identified-hadron production in pp, O-O and Pb-Pb collisions at LHC Run 3 energies with EPOS4

TL;DR

This paper uses the EPOS4 model to predict particle production and collective effects in pp, O-O, and Pb-Pb collisions at LHC energies, revealing system-size-dependent phenomena and the importance of hadronic-phase effects.

Contribution

It introduces EPOS4 predictions for various collision systems, emphasizing non-universal scaling, hadronic effects, and system-size dependencies, providing a baseline for upcoming experimental analyses.

Findings

Charged-particle densities show participant scaling.

Mean transverse momentum varies strongly with system size.

UrQMD is essential for reproducing certain hadronic effects.

Abstract

The observation of collectivity in small and large collision systems challenges our understanding of thermalization and particle production. EPOS4 models this via a dynamical core--corona separation, where high-density regions form a collectively expanding core while low-density regions hadronize via string fragmentation. Its microcanonical core hadronization improves the description of transverse momentum and multiplicity-dependent observables. We present EPOS4 predictions for pp, O-O and Pb-Pb collisions, with and without UrQMD, showing non-universal $\langle p_T\rangle$ scaling, significant hadronic-phase effects, and system-size-dependent $R_{AA}$ suppression. Charged-particle and transverse-energy densities show participant scaling; the transverse energy per charged particle is systematically larger in O--O than in Pb--Pb at comparable participant fraction, indicating a harder effective production in the lighter system. Identified-hadron spectra harden with event multiplicity with mass ordering and increasing core fractions. The mean transverse momentum exhibits a strong system dependence, with the steepest multiplicity evolution in pp, demonstrating that $\langle p_T\rangle$ does not follow universal multiplicity scaling. The $p/\pi$ ratio shows an enhanced intermediate-$p_T$ region; the suppression of the integrated $p/\pi$ at the highest Pb--Pb multiplicities is reproduced only with UrQMD, highlighting hadronic-phase effects. The nuclear modification factor shows sizeable suppression in Pb--Pb and substantial suppression in central O--O collisions. Blast-wave fits exhibit the anti-correlation between $T_{\rm kin}$ and $\langle\beta_T\rangle$, with UrQMD shifting the parameters towards lower $T_{\rm kin}$ and higher $\langle\beta_T\rangle$. These results provide a timely baseline for Run~3 measurements and for constraining the onset of medium-like effects across system size.