Scalings of elliptic flow for a fluid at finite shear viscosity

Within a parton cascade approach we investigate the scaling of the differential elliptic flow v2(pT)v2(pT) with eccentricity ϵxϵx and system size and its sensitivity to finite shear viscosity. We present calculations for shear viscosity to entropy density ratio η/sη/s in the range from 1/4π1/4π up to 1/π1/π, finding that the v2v2 saturation value varies by about a factor 2. Scaling of v2(pT)/ϵxv2(pT)/ϵx is seen also for finite η/sη/s which indicates that it does not prove a perfect hydrodynamical behavior, but is compatible with a plasma at finite η/sη/s. Introducing a suitable freeze-out condition, we see a significant reduction of v2(pT)v2(pT) especially at intermediate pTpT and for more peripheral collisions. This causes a breaking of the scaling for both v2(pT)v2(pT) and the pTpT-averaged v2v2, while keeping the scaling of v2(pT)/〈v2〉v2(pT)/〈v2〉. This is in better agreement with the experimental observations and shows as a first indication that the η/sη/s should be significantly lower than the pQCD estimates. We finally point out the necessity to include the hadronization via coalescence for a definite evaluation of η/sη/s from intermediate pTpT data.