200 A GeV Au+Au collisions serve a nearly perfect quark-gluon liquid

TL;DR

This study extracts the shear viscosity to entropy density ratio of the quark-gluon plasma from elliptic flow measurements in heavy-ion collisions, indicating it is close to a nearly perfect fluid with minimal viscosity.

Contribution

It provides a new method to determine the QGP's viscosity using elliptic flow data and viscous hydrodynamics coupled with microscopic models, with quantified uncertainty.

Findings

The QGP has a shear viscosity to entropy density ratio between 1 and 2.5.

Elliptic flow scaled by eccentricity is a universal function of charged multiplicity.

The QGP behaves as a nearly perfect fluid with minimal viscosity.

Abstract

The specific shear viscosity (eta/s)_QGP of a Quark-Gluon-Plasma (QGP) at temperatures T_c < T < 2T_c is extracted from the centrality dependence of the eccentricity-scaled elliptic flow measured in ultra-relativistic heavy-ion collisions. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find that the eccentricity-scaled elliptic flow is a universal function of charged multiplicity per unit overlap area, (1/S)(dN_ch/dy), that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity. Comparing with measurements we find 1 < (4pi)(eta/s)_QGP < 2.5 where the uncertainty range is dominated by model uncertainties for the eccentricity values used to normalize the measured elliptic flow.