Scaling of soft QGP signatures in relativistic lead, xenon and oxygen collisions in EPOS4

Abstract

The collective expansion and hydrodynamic evolution in heavy-ion collisions is well-established. However, whether femtometer-scale droplets of QGP are produced in small systems at high energies remains a fundamental open question. Analysis of Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, Xe$-$Xe at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV and O$-$O collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.36 TeV using EPOS4 is reported to make predictions and postdictions. The results are compared with ALICE data for Pb$-$Pb and Xe$-$Xe collisions. Charged particle multiplicity (d$N_{\mathrm{ch}}$/d$η$), transverse-momentum ($p_{\mathrm{T}}$) spectra for pions ($π^{\pm}$), kaons ($K^{\pm}$), and protons ($\text{p}(\overline{\text{p}})$) are studied. The inclusion of hadronic afterburner, UrQMD (Ultra-relativistic Quantum Molecular Dynamics) is found to be necessary to correctly describe baryon yields. $p_{\mathrm{T}}$-fluctuations are also studied via normalized $p_{\mathrm{T}}$ correlator, $\sqrt{\langle \langle Δp_{\mathrm{T},i}Δp_{\mathrm{T},j} \rangle \rangle}\mathrm{/} \langle \langle p_{\mathrm{T}} \rangle \rangle$. Lastly, anisotropic flow harmonics ($v_{\mathrm{2}} \{2\}$, $v_{\mathrm{3}} \{2\}$) are computed as a function of $p_{\mathrm{T}}$ and centrality. Since EPOS4 has not been extensively studied for flow observables, this study thereby provides a non-trivial assessment of its collective dynamics. The results are compared with experiment wherever data is available. Taken together, this study provides a unified description of soft observables from Pb$-$Pb through Xe$-$Xe down to O$-$O and offer quantitative guidance on how such collisions may inform of the properties of the QGP.