Study of ⟨ p_{\rm T} ⟩ and its higher moments, and extraction of the speed of sound in Pb-Pb collisions with ALICE

2025/06/12 by ALICE Collaboration
#FOS: Physical sciences #Nuclear Experiment (nucl-ex)

paper · doi:10.48550/arxiv.2506.10394

Abstract

Ultrarelativistic heavy-ion collisions create a quark-gluon plasma (QGP), a hot and dense state of strongly interacting QCD matter. In ultracentral collisions, the QGP volume remains nearly constant event-by-event, while its total entropy can fluctuate due to quantum effects, leading to temperature variations. These features allow the correlation between the mean transverse momentum (⟨ p_T ⟩) of charged hadrons and their multiplicity to serve as a probe of the QGP's speed of sound, c_s. This study extracts c_s by analyzing the relative increase in ⟨ p_T ⟩ with respect to the charged-particle density (⟨ dN_ch/dη⟩) at midrapidity in Pb-Pb collisions at √s_NN = 5.02~TeV, using data from the ALICE detector. Centrality is determined with estimators based on multiplicity and transverse energy, applying a pseudorapidity gap to reduce selection biases. The extracted value of c_s² is found to strongly depend on the employed centrality estimator and ranges between 0.1146 ± 0.0028 (stat.) ± 0.0065 (syst.) and 0.4374 ± 0.0006 (stat.) ± 0.0184 (syst.) in natural units. Additionally, the event-by-event [p_T] distribution is studied through its variance, skewness, and kurtosis. A pronounced decrease in the self-normalized variance and a peak followed by a drop in skewness suggest the suppression of impact-parameter fluctuations in ultracentral collisions. These observations provide new insights into the thermodynamic properties and initial-state fluctuations of the QGP.

Citations

• Ultracentral heavy ion collisions, transverse momentum and the equation of state
• Generic multi-particle transverse momentum correlations as a new tool for studying nuclear structure at the energy frontier
• A generalized hydrodynamizing initial stage for Heavy Ion Collisions
• Relativistic Fluid Dynamics In and Out of Equilibrium -- Ten Years of\n Progress in Theory and Numerical Simulations of Nuclear Collisions
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• Bootstrap Methods: Another Look at the Jackknife

Related

• Assessing the QGP speed of sound in ultra-central heavy-ion collisions with ALICE
• High-\pt processes measured with ALICE at the LHC
• Feasibility of measuring the speed of sound of the quark-gluon plasma from the multiplicity and mean p_T of ultracentral heavy-ion collisions
• Collective phenomena study in small systems using the bulk of particle production in high-multiplicity pp collisions at √(s)= 13 TeV with ALICE
• Probing QGP using local charge fluctuations in heavy-ion collisions