Universal Centrality Dependence of Particle Multiplicities in Heavy-Ion Collisions

Abstract

Starting from the light-cone plasma (LCP) distribution of gluons produced in proton-proton collisions, we derive the invariant charged-particle yield dN/dyd^2pT in heavy-ion collisions. Multiple scattering of partons in the other nucleus leads to pT broadening, which depends on the centrality of the collision. The resulting change of the LCP parameters determines the distributions of charged particles. The charged-particle multiplicity dN/deta at eta=0 normalized to the number of participating nucleons has a universal centrality dependence, common to RHIC and LHC. This universal increase is of geometrical origin. Such a universality can be broken by the energy dependent coherence effect, shadowing and mutual boosting of the saturation scales. However, we found these effects to essentially compensate each other, leaving the centrality dependence of multiplicity approximately universal.