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

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 $⟨p_T⟩$ 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 $⟨p_T⟩$ 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 $T_{\mathrm{kin}}$ and $⟨{\beta }_T⟩$, with UrQMD shifting the parameters towards lower $T_{\mathrm{kin}}$ and higher $⟨{\beta }_T⟩$. These results provide a timely baseline for Run~3 measurements and for constraining the onset of medium-like effects across system size.

Comments: 34 pages, 21 figures