Viscosity information from relativistic nuclear collisions: How perfect is the fluid observed at RHIC?

Relativistic viscous hydrodynamic fits to Brookhaven Relativistic Heavy Ion Collider data on the centrality dependence of multiplicity, transverse, and elliptic flow for s=200  GeV Au+Au collisions are presented. For standard (Glauber-type) initial conditions, while data on the integrated elliptic flow coefficient v2 are consistent with a ratio of viscosity over entropy density up to Ε/sâ‰0.16, data on minimum bias v2 seem to favor a much smaller viscosity over entropy ratio, below the bound from the antiâ€"de Sitter conformal field theory conjecture. Some caveats on this result are discussed.