Effects of a temperature-dependent shear viscosity-to-entropy density ratio on elliptic flow in heavy-ion collisions at RHIC and LHC

We investigate, by using relativistic fluid dynamics, the effects of a temperature-dependent shear viscosity-to-entropy density ratio η/s on transverse momentum spectra and elliptic flow of hadrons in ultrarelativistic heavy-ion collisions. We show that the elliptic flow coefficient in √(sNN) = 200 GeV Au+Au collisions (i.e., at the Relativistic Heavy Ion Collider - RHIC) are insensitive to the value of η/s in the high-temperature quark-gluon plasma (QGP), as long as it has the same minimum value near the QCD phase transition. On the other hand, the elliptic flow is very sensitive to the η/s value of the hadron gas. We find that the sensitivity of the elliptic flow to the high-temperature value of η/s increases with increasing multiplicity and simultaneously the sensitivity to the hadronic viscosity decreases. This makes Pb+Pb collisions at the Large Hadron Collider (LHC) more suitable to extract transport properties of QCD matter at high temperature.