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 $⟨p_t⟩$ 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 $⟨p_t⟩$ is proportional to the speed of sound squared $c_s^2$ of the quark-gluon plasma. Based on the value of $c_s^2$ from lattice QCD, we expect $⟨p_t⟩$ to increase by approximately $18$ MeV between 1\% and 0.001\% centrality in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV.

Comments: 5 pages, 3 figures. v2: Minor revision