Hyrdo + Boltzmann hybrid approach for viscous QCD matter at RHIC and LHC energies

The successes of ideal hydrodynamics in descriibing Relativistic Heavy Ion Collisions (RHIC) data at lower $p_T$ region led to the well known announcement that "RHIC Scientists Serve up Perfect Fluid" (the Quark Gluon Plasma, QGP). In order to answer "How perfect is the QGP fluid?", one needs to extract the QGP viscosity from experimental data. I will report recent progress on a viscous hydrodynamics + hadron cascade hybrid approach and show the extracted QGP viscosity $\eta/s$ from RHIC data with a reliable uncertainty estimate, Then I will discuss the temparature dependent $\eta/s(T)$ at RHIC and LHC energies.