Event-by-event gluon multiplicity, energy density, and eccentricities in ultrarelativistic heavy-ion collisions

Abstract

The event-by-event multiplicity distribution, the energy densities and energy density weighted eccentricity moments ${\ensuremath{\epsilon}}_{n}$ (up to $n=6$) at early times in heavy-ion collisions at both the BNL Relativistic Heavy Ion Collider (RHIC) ($\sqrt{s}=200\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$) and the CERN Large Hardron Collider (LHC) ($\sqrt{s}=2.76\phantom{\rule{0.16em}{0ex}}\mathrm{TeV}$) are computed in the IP-Glasma model. This framework combines the impact parameter dependent saturation model (IP-Sat) for nucleon parton distributions (constrained by HERA deeply inelastic scattering data) with an event-by-event classical Yang-Mills description of early-time gluon fields in heavy-ion collisions. The model produces multiplicity distributions that are convolutions of negative binomial distributions without further assumptions or parameters. In the limit of large dense systems, the $n$-particle gluon distribution predicted by the Glasma-flux tube model is demonstrated to be nonperturbatively robust. In the general case, the effect of additional geometrical fluctuations is quantified. The eccentricity moments are compared to the MC-KLN model; a noteworthy feature is that fluctuation dominated odd moments are consistently larger than in the MC-KLN model.