Quark-gluon plasma at the RHIC and the LHC: Perfect fluid too perfect?

Abstract

Relativistic heavy-ion collisions have reached energies that enable the creation of a novel state of matter termed the quark-gluon plasma. Many observables point to a picture of the medium as rapidly equilibrating and expanding as a nearly inviscid fluid. In this paper, we explore the evolution of experimental flow observables as a function of collision energy and attempt to reconcile the observed similarities across a broad energy regime in terms of the initial conditions and viscous hydrodynamics. If the initial spatial anisotropies for all collision energies from 39 GeV to 2.76 TeV are very similar, we find that viscous hydrodynamics might be consistent with the level of agreement for v2 of unidentified hadrons as a function of pT. However, we predict a strong collision energy dependence for the proton v2(pT). The results presented in this paper highlight the need for more systematic studies and for a re-evaluation of previously reported sensitivities to the early time dynamics and properties of the medium. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.