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

A new robust method to extract the specific shear viscosity (eta/s)(QGP) of a quark-gluon plasma (QGP) at temperatures T-c < T <= 2T(c) from the centrality dependence of the eccentricity-scaled elliptic flow v(2)/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 Au + Au collisions that v(2)/epsilon is a universal function of multiplicity density (1/S)(dN(ch)/dy) that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity epsilon. Comparing with measurements we find 1 < 4 pi(eta/s)(QGP) < 2.5 where the uncertainty range is dominated by model uncertainties for the values of epsilon used to normalize the measured v(2).