Quark-Gluon Plasma at RHIC and the LHC: Perfect Fluid too Perfect?

TL;DR

This paper reviews how relativistic heavy ion collisions create quark-gluon plasma that behaves like a nearly perfect fluid, analyzing flow observables across energies and discussing implications for understanding the medium's properties.

Contribution

It investigates the energy dependence of flow observables and suggests viscous hydrodynamics can explain certain measurements across a wide energy range.

Findings

Initial spatial anisotropies are similar across energies from 39 GeV to 2.76 TeV.

Viscous hydrodynamics can describe v2 of unidentified hadrons as a function of pT.

Proton v2(pT) shows a strong dependence on collision energy.

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 article, 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 are very similar for all collision energies from 39 GeV to 2.76 TeV, 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 a re-evaluation of previously stated sensitivities to the early time dynamics and properties of the medium.