Measurement of correlations among net-charge, net-proton, and net-kaon multiplicity distributions in Pb$-$Pb collisions at $\sqrt{s_\text{NN}}=5.02$ TeV

TL;DR

Using net-proton as a proxy for net-baryon and net-kaon as a proxy for net-strangeness, ALICE measures second-order fluctuations and off-diagonal cumulants of conserved charges in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. The study compares data to HIJING, EPOS LHC, and Thermal-FIST under both grand-canonical and canonical ensembles, finding that local charge conservation in the canonical ensemble with resonance decays best describes the data and allows extraction of a correlation volume around $V_c\approx 2.6$--$2.8\,dV/dy$. Some observables, including ratios of off-diagonal to diagonal cumulants, challenge the model and hint at additional physics such as initial-state fluctuations or magnetic-field effects consistent with lattice QCD predictions. The results support the importance of local conservation and resonance decays at LHC energies and provide qualitative alignment with lattice-QCD expectations for magnetic-field sensitivity while highlighting ongoing modeling challenges.

Abstract

Correlations among conserved quantum numbers, such as the net-electric charge, the net-baryon, and the net-strangeness in heavy-ion collisions, are crucial for exploring the QCD phase diagram. In this paper, these correlations are investigated using net-proton number (as a proxy for the net-baryon), net-kaon number (for the net-strangeness), and net-charged particle number in Pb--Pb collisions at $\sqrt{s_\text{NN}}=5.02$ TeV with the ALICE detector. The observed correlations deviate from the Poissonian baseline, with a more pronounced deviation at LHC energies than at RHIC. Theoretical calculations of the Thermal-FIST hadron resonance gas model, HIJING, and EPOS LHC event generators are compared with experimental results, where a significant impact of resonance decays is observed. Thermal-FIST calculations under the grand canonical and canonical ensembles highlight significant differences, underscoring the role of local charge conservation in explaining the data. Recent lattice QCD studies have demonstrated that the magnetic field generated by spectator protons in heavy-ion collisions affects susceptibility ratios, in particular those related to the net-electric charge and the net-baryon numbers. The experimental findings are in qualitative agreement with the expectations of lattice QCD.

Figures (8)

• Figure 1: Centrality dependence of diagonal cumulants ($\kappa^{2}_{\alpha}$) and off-diagonal cumulants ($\kappa^{11}_{\alpha,\beta}$) for the net-pion, net-kaon, net-proton, and net-charged particle distributions in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. The measurements are shown for two different $p_\mathrm{T}$ ranges. The statistical (systematic) uncertainties are represented by vertical bars (boxes).
• Figure 2: Centrality dependence of correlations $C_\mathrm{Q,p}$ (top), $C_\mathrm{Q,K}$ (middle), and $C_\mathrm{p,K}$ (bottom) in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. The measurements are shown for two different $p_\mathrm{T}$ ranges in left and right columns. The predictions from HIJING Wang:1991hta, EPOS LHC Pierog:2013ria, and Thermal-FIST (TheFIST) Vovchenko:2019pjl model calculations with the grand canonical ensemble (GCE) and canonical ensemble (CE) formulation are denoted by colored lines. In the CE calculations, the electric charge (Q), baryon number (B), and strangeness (S) are conserved in a correlation volume of $V_c=3\mathrm{d}V/\mathrm{d}y$. The statistical (systematic) uncertainties are represented by vertical bars (boxes), and the dashed line corresponds to the Poisson baseline.
• Figure 3: (Color online) Correlations $C_\mathrm{p,K}$ (left), $C_\mathrm{Q,p}$ (middle), and $C_\mathrm{Q,K}$ (right) are shown as a function of centrality in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. Colored lines represent Thermal-FIST (TheFIST) Vovchenko:2019pjl model calculations for the canonical ensemble (CE) with different correlation volumes ($V_{c}$). The statistical (systematic) uncertainties are represented by vertical bars (boxes).
• Figure 4: Correlations $C_\mathrm{p,K}$ (left), $C_\mathrm{Q,p}$ (middle), and $C_\mathrm{Q,K}$ (right) are shown as a function of centrality in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. Colored lines represent Thermal-FIST (TheFIST) Vovchenko:2019pjl model calculations for canonical ensemble (CE) with (w) and without (w/o) resonance contribution. The statistical (systematic) uncertainties are represented by vertical bars (boxes).
• Figure 5: Centrality dependence of $\kappa^{11}_\mathrm{Q,p}/\kappa^{2}_\mathrm{Q}$ (left) and $\kappa^{11}_\mathrm{Q,K}/\kappa^{2}_\mathrm{p}$ (right) in Pb--Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. Colored lines represent Thermal-FIST (TheFIST) Vovchenko:2019pjl model calculations for canonical ensemble (CE) with different correlation volumes ($V_{c}$). The statistical (systematic) uncertainties are represented by vertical bars (boxes).