Elliptic flow and shear viscosity from a beam energy scan

We study within a relativistic transport approach the impact of a temperature dependent shear viscosity to entropy density ratio, η/s(T), on the build-up of the elliptic flow, v2, a measure of the angular anisotropy in the particle production. Beam Energy Scan from √sNN = 62.4GeV at RHIC up to 2.76TeV at LHC has shown that the v2(pT ) as a function of the transverse momentum pT appears to be nearly invariant with energy. We show that such a surprising behavior is determined by a rise and fall of η/s(T) with a minimum at T ∼ T_c, as one would expect if the matter undergoes a phase transition or a cross-over.