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

Fernando G Gardim,Giuliano Giacalone,Jean-Yves Ollitrault

doi:10.1016/j.physletb.2020.135749

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

Highlights

We predict a new phenomenon to be observed in experimental data on heavy-ion collisions: a rise of the mean transverse momentum of charged hadrons, pt, in ultracentral collisions
The total entropy in the quark-gluon plasma is proportional to the multiplicity, at constant volume, the entropy density, s, is itself proportional to the multiplicity, and varies by a few percent
In ultracentral collisions the temperature increases as a function of the multiplicity, which in turn implies a rise of the mean transverse momentum of charged hadrons [1], pt, observed in the final state, due to tight correlation with the temperature [2]

Summary

Introduction

We predict a new phenomenon to be observed in experimental data on heavy-ion collisions: a rise of the mean transverse momentum of charged hadrons, pt , in ultracentral collisions. Recent experimental analyses [3] seem to contradict this prediction: pt varies by less than 0√.2% in the 0-20% centrality range in Pb+Pb collisions at sNN = 5.02 TeV These analyses use wide centrality bins, while, as we shall see, the rise is only expected in ultracentral events. The observed flatness of pt implies that even a modest rise in the ultracentral range [4, 5, 6] will be easy to identify This new phenomenon is expected to be driven by the speed of sound cs, which can be determined experimentally in ultrarelativistic heavy-ion collisions.

Quantitative analysis

Findings

Analytic model