200 A GeV Au + Au Collisions Serve a Nearly Perfect Quark-Gluon Liquid

A new robust method to extract the specific shear viscosity $(ensuremath{eta}/s{)}_{mathrm{QGP}}$ of a quark-gluon plasma (QGP) at temperatures ${T}_{c}<Tensuremath{lesssim}2{T}_{c}$ from the centrality dependence of the eccentricity-scaled elliptic flow ${v}_{2}/ensuremath{epsilon}$ measured in ultrarelativistic heavy-ion collisions is presented. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find for 200 $A$ GeV $mathrm{Au}+mathrm{Au}$ collisions that ${v}_{2}/ensuremath{epsilon}$ is a universal function of multiplicity density $(1/S)(d{N}_{mathrm{ch}}/dy)$ that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity $ensuremath{epsilon}$. Comparing with measurements we find $1<4ensuremath{pi}(ensuremath{eta}/s{)}_{mathrm{QGP}}<2.5$ where the uncertainty range is dominated by model uncertainties for the values of $ensuremath{epsilon}$ used to normalize the measured ${v}_{2}$.