Centrality dependence of strange particle production in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

TL;DR

This paper investigates how strange and multi-strange hadron production in Pb–Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV depends on centrality, using the largest Run 2 data set to measure $p_T$ spectra and yields for ${\rm K^0_S}$, $\Lambda$, $\Xi$, and $\Omega$ across nine centrality bins. The analysis compares to EPOS 4 and interprets the results in the framework of hydrodynamic flow and core–corona hadronization, highlighting a multiplicity-driven evolution of strangeness that is largely independent of system or energy within the studied range. The findings show spectrum hardening with centrality, a centrality-dependent $\Lambda$/K^0_S peak at intermediate $p_T$, and yield ratios to pions that rise with multiplicity and saturate in Pb–Pb, providing stringent constraints for QGP and hadronization models. Overall, the work establishes a high-precision baseline for tuning theoretical models and for exploring the universal features of strangeness production across collision systems and energies.

Abstract

The centrality dependence of strange ($K_S^0$, $Λ+ \barΛ$) and multi-strange ($Ξ^- + \bar{Ξ^+}$, $Ω^- + \barΩ^+$) hadron production is measured by ALICE in the LHC lead-lead (Pb-Pb) collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$~TeV, using the full data set collected during the LHC Run 2 campaign in the years 2015 and 2018. This is the largest heavy-ion data set analyzed to date at the LHC, and it allows for the extraction of transverse momentum ($p_T$) spectra and $p_T$-integrated yields with unprecedented precision, over a broad range of charged particle multiplicity densities ($\langle dN_{ch}/dη\rangle_{|η|<0.5}$), probing regions where smaller collision system (pp and p-Pb) results are also available. The $p_T$ spectra evolve with centrality, featuring higher $\langle p_T \rangle$ in central events for all particles. The $Λ/K_S^0$ ratio exhibits the distinctive baryon-to-meson enhancement in the intermediate $p_T$ region, with a maximum which is shifted to larger $p_T$ for more central collisions. The hadron-to-pion yield ratios are presented as a function of $\langle dN_{ch}/dη\rangle_{|η|<0.5}$ and compared to results from different collision systems and energies. A smooth connection from pp to Pb-Pb is observed, thus demonstrating that collision system or energy do not play a role in the multiplicity evolution of this observable. The previously reported enhancement of strangeness production in the multiplicity range probed in pp and p_Pb collisions saturates in the multiplicity range of Pb-Pb data. These results constitute a key test bench for theoretical models and a first comparison to the EPOS~4 generator is presented.

Figures (5)

• Figure 1: Invariant mass distributions for (top left) ${\rm K}^{0}_{\rm{S}}\xspace$ (top right) $\Lambda\xspace$ (bottom left) $\Xi^-\xspace$ and (bottom right) $\Omega^-\xspace$ for the lowest transverse momentum intervals accessed by the analysis. The distributions for the most central (red full markers) and most peripheral (blue open markers) class of events are reported. The dashed lines illustrate the fits used for signal extraction.
• Figure 2: (left) Acceptance times efficiency ($\mathrm{Acc \times Eff}\xspace$) as a function of $p_{\rm T}$ for the reconstruction of ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi^-$, $\Omega^-$ (full markers) and respective antiparticles (open markers). The 0--90% centrality class is displayed. (right) Ratio of $\Lambda$ coming from charged and neutral $\Xi$ baryon decays to the inclusive $\Lambda$ yield as a function of $p_{\rm T}$. Full and hollow markers are for matter and antimatter, respectively. The 0--90% centrality class is displayed.
• Figure 3: ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi$, and $\Omega$ transverse momentum spectra in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeemV for different event centrality intervals. Error bars show the statistical uncertainty, while boxes represent the total systematic uncertainty. For each centrality class, predictions from the EPOS 4 EPOS4:strangeness model are reported as solid lines, with the corresponding transparent area accounting for the statistical uncertainty on the model. Scale factors are applied to both the results and the model to improve visibility.
• Figure 4: (left) $\langle p_{\mathrm{T}}\rangle$ as a function of $\langle\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta\xspace\rangle$ for ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi$, and $\Omega$ particle production in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeemV at midrapidity (square markers). Results from pp silviapp5_TBU and p--Pb Alice_strpPb5Alice_multistrpPb5 collisions at the same center-of-mass energy are also reported as circles and open diamonds, respectively. Error bars correspond to the statistical uncertainty, while boxes correspond to the total systematic uncertainty. Solid lines are predictions from the EPOS 4 EPOS4:strangeness model. (right) $\Lambda$/${\rm K}^{0}_{\rm{S}}$ ratio as a function of $p_{\rm T}$ for the 0--10% most central (red), 40--50% intermediate (green) and 80--90% most peripheral (blue) centrality classes. Error bars correspond to the statistical uncertainty, while boxes correspond to the total systematic uncertainty. EPOS 4 EPOS4:strangeness predictions, scaled to the total particle yields in the corresponding centrality class, are reported as solid lines.
• Figure 5: ${\rm K}^{0}_{\rm{S}}$, $\Lambda+\overline{\Lambda}$, $\Xi^-+\overline{\Xi}^+$, and $\Omega^-+\overline{\Omega}^+$ particle yield ratios to the pion yield in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeemV (squares), compared to results in pp collisions at $\sqrt{s}~=~7$ TeemV ALICEstrPP7 and $\sqrt{s}~=~13$ TeemV ALICEstrPP13 (hollow and full circles, respectively), in p--Pb collisions at $\sqrt{s}~=~5.02$ TeemV ALICE_cascpPb5 (hollow diamonds), and in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}~=~2.76$ TeemV ALICE_v0PbPb276ALICE_cascPbPb276 (stars). Error bars show the statistical uncertainty, while boxes represent the total systematic uncertainty. EPOS 4 EPOS4:strangeness predictions are reported for the top Pb--Pb energy as solid lines.