Extracting the speed of sound in quark–gluon plasma with ultrarelativistic lead–lead collisions at the LHC
Journal
Reports on Progress in Physics
Journal Volume
87
Journal Issue
7
Start Page
077801
ISSN
0034-4885
1361-6633
Date Issued
2024-06-20
Author(s)
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 ±0.002(stat) ±0.016(syst) in natural units. The effective medium temperature, estimated using the mean transverse momentum, is 219 ±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.
Subjects
CMS
QCD equation of state
quark-gluon plasma
speed of sound
ultra-central
Publisher
IOP Publishing
Type
journal article