The viscosity to entropy density ratio from PHENIX data on single electron production

The PHENIX experiment has measured electrons from the decay of hadrons carrying heavy flavor (charm or bottom quarks) in p + p and Au + Au collisions at √sNN = 200 GeV at the Relativistic Heavy Ion Collider (RHIC). The strong damping of heavy-flavor motion through the medium is reflected in a substantial suppression at high pT and a large azimuthal anisotropy υ2 of such electrons in Au + Au collisions at RHIC. By requiring a simultaneous description of the nuclear modification factor RAA(pT) and the azimuthal anisotropy υ2(pT) in transport models, the viscosity to entropy density ratio of the medium can be estimated as η/s ∼ (1.3-2)(1/4π), which is close to the conjectured lower quantum bound and, therefore, near a perfect fluid