Long-range transverse momentum correlations and radial flow in Pb$-$Pb collisions at the LHC with ALICE

Abstract

This Letter presents measurements of long-range transverse-momentum correlations using a new observable, $v_{0}(p_\mathrm{T})$, serving as a probe of event-by-event radial-flow fluctuations, the underlying radial expansion, and the medium properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low $p_\mathrm{T}$, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher $p_\mathrm{T}$ ($> 3$ GeV/$c$), protons exhibit larger $v_{0}(p_\mathrm{T})$ than pions and kaons, in agreement with expectations from quark-recombination models. Comparisons to viscous hydrodynamic calculations with varying bulk viscosity and equation of state demonstrate the sensitivity of the $v_{0}(p_\mathrm{T})$ observable to these key medium properties. The findings establish $v_{0}(p_\mathrm{T})$ as a valuable addition to the set of observables used in Bayesian analyses for extracting the transport properties and constraining the equation of state of strongly interacting matter, while also helping to systematically explore its sensitivity and impact within such global studies.

Figures (8)

• Figure 1: $v_{0}(p_\mathrm{T})$ of inclusive charged particles shown as a function of $p_\mathrm{T}$ in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV for centrality intervals 10$-$20% (a), 30$-$40% (b), and 60$-$70% (c). The measurements are compared to expectations from HIJING Wang:1991hta and IP-Glasma+MUSIC+UrQMD Mantysaari:2024uwn models. $v_{0}(p_\mathrm{T})/v_{0}$ of inclusive charged particles shown as a function of $p_\mathrm{T}$ for the centrality intervals (d). The statistical (systematic) uncertainties are represented by vertical bars (boxes).
• Figure 2: $v_{0}(p_\mathrm{T})$ of pions ($\pi^{\pm}$), kaons ($\mathrm{K}^{\pm}$), and protons ($\mathrm{p}(\bar{\mathrm{p}})$) shown as a function of $p_\mathrm{T}$ in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV for centrality intervals 10$-$20% (a), 30$-$40% (b), and 60$-$70% (c). The measurements are compared to results from HIJING Wang:1991hta and IP-Glasma+MUSIC+UrQMD Mantysaari:2024uwn models. The statistical (systematic) uncertainties are represented by vertical bars (boxes). The bottom panels show the (Data-Model)/$\sigma$, representing the deviation between the experimental data and model predictions, normalized by the uncertainty.
• Figure 3: Comparison of $v_{0}(p_\mathrm{T})$ for inclusive charged particles in 10$-$20% central Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with IP-Glasma+MUSIC+UrQMD Mantysaari:2024uwn calculations: variations in transport coefficients, $\eta/s$ and $\zeta/s$ (a), different equations of state from Ref. Gong:2024lhq (b), and initial conditions (IC) with (w/) and without (w/o) sub-nucleonic fluctuations (c). The statistical (systematic) uncertainties are represented by vertical bars (boxes).
• Figure A.1: $v_{0}(p_\mathrm{T})$ of pions (a), kaons (b), and protons (c) shown as a function of $p_\mathrm{T}$ using blast-wave model parameters from Ref. ALICE:2019hno. The open marker represents results for a slightly smaller value of $\langle\beta_\mathrm{T}\rangle$.
• Figure A.2: $v_{0}(p_\mathrm{T})$ of inclusive charged particles shown as a function of $p_\mathrm{T}$ for centrality intervals 10$-$20% (a), 30$-$40% (b), and 60$-$70% (c) for varying pseudorapidity gap ($\Delta\eta$) in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The error bars represent statistical uncertainties. The bottom panel presents the ratio relative to the results for $\Delta\eta = 0$.