Assessing the speed of sound in Pb–Pb collisions with ALICE
ALICE Collaboration
Report number: ALICE-PUBLIC-2024-002
Ultrarelativistic heavy-ion collisions produce a state of hot and dense strongly interacting QCD matter called quark--gluon plasma (QGP). On an event-by-event basis, the volume of the QGP in ultra-central collisions is mostly constant, while its total entropy can vary significantly, leading to variations in the temperature of the system. Exploiting this unique feature of ultra-central collisions, allows to interpret the correlation of the mean transverse momentum ⟨pT⟩ of produced charged hadrons and the number of charged hadrons as a measure for the the speed of sound, cs. The speed of sound, cs, which is related to the speed at which compression waves travel in a certain type of medium, in this case the QGP, is determined by fitting the relative increase of the ⟨pT⟩ with respect to the relative change of the average charged-particle density (⟨dNch/dη⟩) measured at midrapidity. This study reports the ⟨pT⟩ of charged particles in ultra-central Pb-Pb collisions at center-of-mass energy of 5.02 TeV per nucleon pair using the ALICE detector. Different centrality estimators based on charged-particle multiplicity or the transverse energy of the event are used to select ultra-central collisions. By ensuring a pseudorapidity gap between the region used to define the centrality and the region to perform the measurement, the influence of biases and their potential effects on the rise of the mean transverse momentum is tested. The measured cs2 is found to strongly depend on the type of the centrality estimator and ranges between 0.113±0.003(stat)±0.007(syst) and 0.438±0.001(stat)±0.019(syst) in natural units.
