The mean transverse momentum of ultracentral heavy-ion collisions: A new probe of hydrodynamics

Abstract

We predict that the mean transverse momentum of charged hadrons 〈pt〉 rises as a function of the charged-particle multiplicity in ultracentral nucleus-nucleus collisions. We explain that this phenomenon has a simple physical origin and represents an unambiguous prediction of the hydrodynamic framework of heavy-ion collisions. We argue that the relative increase of 〈pt〉 is proportional to the speed of sound squared cs2 of the quark-gluon plasma. Based on the value of cs2 from lattice QCD, we expect 〈pt〉 to increase by approximately 18 MeV between 1% and 0.001% centrality in Pb+Pb collisions at sNN=5.02 TeV