Elliptic flow in √s=200 GeV Au+Au collisions and √s=2.76 TeV Pb+Pb collisions: Insights from viscous hydrodynamics plus hadron cascade hybrid model

Using the newly developed hybrid model VISHNU, which connects viscous hydrodynamics with a hadron cascade model, we study the differential and integrated elliptic flow v(2) at different centrality bins for 200 A GeV Au + Au collisions and 2.76 A TeV Pb + Pb collisions. We find that the average quark-gluon plasma (QGP)-specific shear viscosity eta/s slightly increases from Relativistic Heavy Ion Collider to Large Hadron Collider (LHC) energies. However, a further study assuming different temperature dependencies for (eta/s)(QGP) shows that one cannot uniquely constrain the form of (eta/s)(QGP) (T) by fitting the spectra and v(2) alone. Based on our current understanding, the question whether the QGP fluid is more viscous or more perfect in the temperature regime reached by LHC energies is still open.