Anisotropies in momentum space at finite Shear Viscosity in ultrarelativistic heavy-ion collisions
Abstract
Within a parton cascade we investigate the dependence of anisotropies in momentum space, namely the elliptic flow v2=<cos(2φ)> and the v4=<cos(4φ)>, on both the finite shear viscosity η and the freeze-out (f.o.) dynamics at the RHIC energy of 200 AGeV. In particular it is discussed the impact of the f.o. dynamics looking at two different procedures: switching-off the collisions when the energy density goes below a fixed value or reducing the cross section according to the increase in η/s from a QGP phase to a hadronic one. We address the relation between the scaling of v2(pT) with the eccentricity εx and with the integrated elliptic flow. We show that the breaking of the v2(pT)/εx scaling is not coming mainly from the finite η/s but from the f.o. dynamics and that the v2(pT) is weakly dependent on the f.o. scheme. On the other hand the v4(pT) is found to be much more dependent on both the η/s and the f.o. dynamics and hence is indicated to put better constraints on the properties of the QGP. A first semi-quantitative analysis show that both v2 and v4 (with the smooth f.o.) consistently indicate a plasma with 4π η/s 1-2.