Speaker
Description
A hot and dense quark–gluon medium is observed to be created in ultrarelativistic nuclear collisions, exhibiting striking collective flow behavior, with fluid-like properties well described by relativistic hydrodynamics, with minimal viscous dissipation. The characteristics of this medium are under intense investigation, and in particular, its thermodynamical properties. In this context, the measurement of the speed of sound provides a key probe, as it reflects the medium pressure response to variations in energy density. This observable has been investigated by studying the correlations between the average transverse momentum, p_T, and the event multiplicity of charged particles, N_{ch}, produced in head-on PbPb collisions at sqrt{s_{NN}} = 5.02 TeV, using data with integrated luminosity of 0.607 nb^{-1}, obtained by the CMS experiment at the CERN LHC. The resulting values for the speed of sound and temperature are the most precise to date. They are consistent with lattice QCD predictions and provide direct evidence for the formation of a deconfined, strongly interacting matter in such extreme conditions. More recently, CMS has extended the measurement of the speed of sound to pPb collisions, using data corresponding to integrated luminosities of 186.0 nb^{-1} at sqrt{s_{NN}} = 8.16 TeV and 0.509 nb^{-1} at sqrt{s_{NN}} = 5.02 TeV. The extracted values are found to be consistent with lattice QCD predictions within uncertainties and are also compared with theoretical models.This investigation is motivated by previous LHC observations of multiparticle correlations in small colliding systems (pp and pPb), which revealed collective behavior reminiscent of that seen in heavy-ion collisions. This talk presents the assembled results for PbPb and pPb collisions in this broader context.
