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Extracting the speed of sound in the strongly interacting matter created in ultrarelativistic lead-lead collisions at the LHC

The CMS collaboration
Hayrapetyan, Aram , Tumasyan, Armen , Adam, Wolfgang , Andrejkovic, Janik Walter , Bergauer, Thomas , Chatterjee, Suman , Damanakis, Konstantinos , Dragicevic, Marko , Hussain, Priya Sajid , Jeitler, Manfred
Rept.Prog.Phys. 87 (2024) 077801, 2024.
https://doi.org/10.17182/hepdata.146016

Abstract (data 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 extracted the speed of sound in this medium created 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 $0.607 \mathrm{nb}^{-1}$. 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 of the speed of light, with its squared value being $0.241\pm0.002 (\mathrm{stat})\pm0.016 (\mathrm{syst})$ in natural units at a medium temperature of $219\pm8(\mathrm{syst})\mathrm{MeV}$, precisely aligning with predictions from lattice quantum chromodynamic (QCD) calculations. This result provides a stringent constraint on the equation of state of the created medium and a direct evidence for a deconfined QCD phase being attained in relativistic nuclear collisions.

  • Figure 2

    Data from Figure 2

    10.17182/hepdata.146016.v1/t1

    The event fraction distribution as a function of the charged-particle multiplicity, $N_{\mathrm{ch}}$, within the kinematic range of $|\eta|<0.5$ and extrapolated...

  • Figure 3

    Data from Figure 3

    10.17182/hepdata.146016.v1/t2

    The average transverse momentum of charged particles, $\langle p_{\mathrm{T}}\rangle$, as a function of the charged-particle multiplicity, $N_{\mathrm{ch}}$, within the kinematic...

  • Figure 4

    Data from Figure 4

    10.17182/hepdata.146016.v1/t3

    The speed of sound, $c^2_{\mathrm{s}}$, as a function of the effective temperature, $T_{\mathrm{eff}}$, with the CMS data point obtained from...

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