Perfect fluidity of the quark–gluon plasma core as seen through its dissipative hadronic corona

Abstract

The agreement of hydrodynamic predictions of differential elliptic flow and radial flow patterns with Au + Au data at s N N = 200 GeV is one of the main lines of evidence suggesting the nearly perfect fluid properties of the strongly coupled quark–gluon plasma, sQGP, produced at RHIC. We study the sensitivity of this conclusion to different hydrodynamic assumptions on hadro-chemical and thermal freezeout after the sQGP hadronizes. We show that if chemical freezeout occurs at the hadronization time, as required to reproduce the observed hadron yields, then, surprisingly, the differential elliptic flow, v 2 ( p T ) , for pions continues to increase with proper time in the late hadronic phase until thermal freezeout and leads to a discrepancy with the v 2 ( p T ) data. In contrast, if both hadro-chemical and thermal equilibrium are maintained past the hadronization point, then the mean transverse momentum per pion increases in a way that accidentally preserves v 2 ( p T ) from the sQGP phase in agreement with the data, but at the cost of the agreement with the observed hadronic yields. In order that all the data on (1) hadronic ratios, (2) radial flow, as well as (3) differential elliptic flow be reproduced, the sQGP core must expand with a minimal viscosity, η ≈ T c 3 , that is however even greater than the viscosity, η H ≈ T / σ H , of its hadronic corona. However, because of the large entropy density difference of the two phases of QCD matter, the larger viscosity in the sQGP phase leads to nearly perfect fluid flow in that phase while the smaller entropy density of the hadronic corona strongly hinders the applicability of Euler hydrodynamics in that phase. The “perfect fluid” property of the sQGP is thus not due to a sudden reduction of the viscosity at the critical temperature T c , but to a sudden increase of the entropy density of QCD matter and is therefore an important signature of deconfinement.