Exploring Thermalization and Multi-Freeze-Out Effects in Pb-Pb collisions Based on Tsallis pT Distributions

TL;DR

This paper analyzes transverse-momentum distributions of various particles in Pb-Pb collisions at 2.76 TeV using Tsallis distributions, revealing how temperature, non-extensivity, and flow vary with collision centrality and particle mass.

Contribution

It introduces a detailed analysis of freeze-out parameters and collective effects in heavy-ion collisions using Tsallis fits, highlighting mass-dependent freeze-out and centrality evolution.

Findings

Temperature T increases with centrality.

Non-extensive parameter q decreases with centrality.

Mass dependence supports multi-freeze-out scenario.

Abstract

This study investigates transverse-momentum (pT) distributions of pi-, pi+, K-, K+, p, pbar, K0s, and Lambda in several centrality classes of Pb-Pb collisions at sqrt(sNN) = 2.76 TeV. The measured spectra are analyzed with the Tsallis non-extensive distribution, from which the effective temperature T, non-extensive parameter q, and the mean transverse momentum mean_pT are extracted for each particle species and centrality interval. To disentangle thermal and collective effects, the mean kinetic freeze-out temperature T0 is obtained from the intercept of the T-versus-mass relation, while the average transverse flow velocity betaT is extracted from the slope of mean_pT versus the mean moving mass for pions, kaons, and protons. The results show that T increases and q decreases with increasing centrality, indicating a hotter and more equilibrated system in central collisions. A clear mass dependence of T supports a multi-freeze-out scenario, with heavier particles decoupling earlier. Both T0 and betaT rise from peripheral to mid-central collisions before saturating toward central events, which may suggest the onset of collective behavior or changes in freeze-out dynamics. These observations provide new insights into the thermal and dynamical properties of the medium created in heavy-ion collisions at the LHC.