The QGP shear viscosity -- elusive goal or just around the corner?
  With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise (O(25%)) extraction of the QGP shear viscosity (eta/s)_QGP from heavy-ion elliptic flow data is possible if the initial eccentricity of the collision fireball is known with <5% accuracy. At this point, eccentricities from initial state models differ by up to 20%, leading to an O(100%) uncertainty for (eta/s)_QGP. It is shown that a simultaneous comparison of elliptic and triangular flow, v_2 and v_3, puts strong constraints on initial state models and can largely eliminate the present uncertainty in (eta/s)_QGP. The variation of the differential elliptic flow v_2(p_T) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model. 
Chun Shen, Steffen A Bass, Tetsufumi Hirano, Pasi Huovinen, Zhi Qiu,   Huichao Song and Ulrich W Heinz
nucl-th hep-ph nucl-ex