Extracting the speed of sound in the strongly interacting matter created in relativistic nuclear collisions

Ultrarelativistic nuclear collisions yield 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, we extracted the speed of sound in this medium utilizing data from $0.607~\mathrm{nb}^{-1}$ of lead-lead collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=5.02~\mathrm{TeV}$, recorded by the CMS experiment in 2018 at the CERN-LHC. This is the most accurate measurement of this intrinsic characteristic to date and is performed using a new hydrodynamic probe in collisions with large overlap of the two lead nuclei. Our findings reveal that the squared speed of sound in this matter is $0.241\pm0.002~(\text{stat})\pm0.016~(\text{syst})$ times the squared speed of light at an effective medium temperature of $219\pm8~(\text{syst})~\mathrm{MeV}$, precisely aligning with predictions from lattice quantum chromodynamic calculations. This result provides the most stringent and direct constraints on the deconfined QCD phase attained by the hot and dense matter created in these collisions.