Anisotropies in momentum space at finite shear viscosity in ultrarelativistic heavy-ion collisions

Within a parton cascade we investigate the dependence of anisotropies in momentum space, namely the elliptic flow v2=〈cos(2ϕ)〉v2=〈cos(2ϕ)〉 and the v4=〈cos(4ϕ)〉v4=〈cos(4ϕ)〉, on both the finite shear viscosity ηη and the freeze-out (f.o.) dynamics at the RHIC energy of 200 GeV. In particular the impact of the f.o. dynamics is discussed 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/sη/s from a QGP phase to a hadronic one. We address the relation between the scaling of v2(pT)v2(pT) with the eccentricity ϵxϵx and with the integrated elliptic flow. We show that the breaking of the v2(pT)/ϵxv2(pT)/ϵx scaling is not coming mainly from the finite η/sη/s but from the f.o. dynamics and that the v2(pT)v2(pT) is weakly dependent on the f.o. scheme. On the other hand the v4(pT)v4(pT) is found to be much more sensitive to both the η/sη/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 shows that both v2v2 and v4v4 (with the smooth f.o.) consistently indicate a plasma with 4πη/s∼1−24πη/s∼1−2.