Extracting the QGP viscosity from RHIC data—a status report from viscous hydrodynamics

We report recent progress on causal viscous hydrodynamics for relativistic heavy-ion collisions. For fixed specific shear viscosity η/ s , uncertainties in the elliptic flow arising from initial conditions, equation of state, bulk viscosity and numerical viscosity, and the treatment of the highly viscous hadronic stage and freeze-out procedure are analysed. A comparison of current viscous hydrodynamic results with experimental data yields a robust upper limit .