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

TL;DR

This work uses the EPOS4 framework with a dynamical core–corona separation, microcanonical core hadronization, and optional UrQMD to predict global observables and identified-hadron production in pp, O–O, and Pb–Pb collisions at LHC Run 3 energies. It demonstrates non-universal $\langle p_T\rangle$ scaling, notable hadronic-phase effects, and system-size–dependent $R_{AA}$ suppression, driven by the evolving balance between core (hydrodynamic) and corona (string fragmentation) contributions. Key findings include harder per-particle production in the lighter O–O system at matched participant fractions, multiplicity-driven spectral hardening with mass ordering, and a pronounced UrQMD-related suppression in high-multiplicity Pb–Pb that feeds into the kinetic freeze-out parameters. The results establish a baseline for Run 3 and offer insight into the onset of medium-like effects across system sizes, emphasizing the importance of late-stage hadronic dynamics and core–corona composition in shaping observables.

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 $p/π$ ratio shows an enhanced intermediate-$p_T$ region; the suppression of the integrated $p/π$ 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 $\langleβ_T\rangle$, with UrQMD shifting the parameters towards lower $T_{\rm kin}$ and higher $\langleβ_T\rangle$. These results provide a timely baseline for Run~3 measurements and for constraining the onset of medium-like effects across system size.

Figures (21)

• Figure 1: (Color online) (left) Pseudorapidity density of primary charged particles for different centrality classes over a broad $\eta$ range in Pb-Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV and (right) Transverse momentum spectra of pions, kaons, and (anti-)protons in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV for 0-5$\%$ centrality class, compared with ALICE PhysRevC.101.0449072017567.
• Figure 2: (Color online) (a-c) Pseudorapidity density of primary charged particles for different centrality classes over a broad $\eta$ range in pp collisions at $\sqrt{s} = 13.6$ TeV and O-O and Pb–Pb collisions at $\sqrt{s_{NN}}$ = 5.36 TeV, and (d) Average charged particle multiplicity as a function of centrality percentile, compared with ALICE 2019alicecollaboration2025centralitydependencechargedparticlepseudorapidity.
• Figure 3: (Color online) (Left) Charged-particle multiplicity density at midrapidity, $\frac{dN_{ch}}{d\eta}$, as a function of the number of participating nucleons, $\langle N_{\text{part}}\rangle$, for O–O and Pb–Pb collisions at $\sqrt{s_{NN}}$ = 5.36 TeV, compared with ALICE data alicecollaboration2025centralitydependencechargedparticlepseudorapidity and (Right) The same observable shown as a function of the scaled participant fraction, $\langle N_{\text{part}}\rangle/2A$.
• Figure 4: (Color online) (left) Midrapidity transverse energy density $\langle dE_T/d\eta\rangle$, and (right) Transverse energy per charged particle, $\langle dE_T/d\eta\rangle$)/($\langle dN_{ch}/d\eta\rangle$, shown as a function of the scaled average participant fraction, $\langle N_{\text{part}}\rangle$/2A, for O-O and Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.36 TeV. Results are compared to available ALICE measurements PhysRevC.94.034903.
• Figure 5: (Color online) Transverse momentum spectra of identified charged hadrons $\pi^{\pm}$, K$^{\pm}$ and p($\bar{p}$) in (top) pp collisions at $\sqrt{s}$ = 13.6 TeV and in (bottom) O-O collisions at $\sqrt{s_{NN}}$ = 5.36 TeV for several multiplicity or centrality classes from EPOS4. For each systems, the upper panels show the $p_T$-differential yields, while the lower panels display the ratios of the spectra in each multiplicity class with respect to the corresponding minimum-bias reference.