Extracting the speed of sound in quark-gluon plasma with ultrarelativistic lead-lead collisions at the LHC

Abstract

Ultrarelativistic nuclear collisions create a strongly interacting state of hot and dense quark-gluon matter that exhibits a remarkable collective flow behavior with minimal viscous dissipation. To gain deeper insights into its intrinsic nature and fundamental degrees of freedom, we determine the speed of sound in an extended volume of quark-gluon plasma using lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data were recorded by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 0.607 nb${}^{-1}$. The measurement is performed by studying the multiplicity dependence of the average transverse momentum of charged particles emitted in head-on PbPb collisions. Our findings reveal that the speed of sound in this matter is nearly half the speed of light, with a squared value of 0.241 $\pm$ 0.002 (stat) $\pm$ 0.016 (syst) in natural units. The effective medium temperature, estimated using the mean transverse momentum, is 219 $\pm$ 8 (syst) MeV. The measured squared speed of sound at this temperature aligns precisely with predictions from lattice quantum chromodynamic (QCD) calculations. This result provides a stringent constraint on the equation of state of the created medium and direct evidence for a deconfined QCD phase being attained in relativistic nuclear collisions.

Replaced to correct one number in the previous version. All the figures and tables, including additional supplementary figures, can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-23-003 (CMS Public Pages)

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Cited by in corpus (13)

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• Novel Scalings of Neutron Star Properties from Analyzing Dimensionless Tolman--Oppenheimer--Volkoff Equations
• Imaging nuclear shape through anisotropic and radial flow in high-energy heavy-ion collisions
• Analytical insights into the interplay of momentum, multiplicity and the speed of sound in heavy-ion collisions
• Impact parameter dependence of anisotropic flow: Bayesian reconstruction in ultracentral nucleus-nucleus collisions
• Study of $⟨p_{\mathrm{T}}⟩$ and its higher moments, and extraction of the speed of sound in Pb-Pb collisions with ALICE
• Feasibility of measuring the speed of sound of the quark-gluon plasma from the multiplicity and mean $p_T$ of ultracentral heavy-ion collisions
• Bayesian reconstruction of anisotropic flow fluctuations at fixed impact parameter
• The size of the quark-gluon plasma in ultracentral collisions: impact of initial density fluctuations on the average transverse momentum
• Rapidity dependence of mean transverse momentum fluctuation and decorrelation in baryon-dense medium
• High-order fluctuations of temperature in hot QCD matter
• Geometric Bias and Centrality Dependence of Jet Quenching in High-Energy Nuclear Collisions
• Blast-wave-Tsallis-power model for $p_{\mathrm{T}}$-spectra and elliptic flow $v_2$ of hadrons in collisions of identical nuclei at energies available at the Large Hadron Collider