What the collective flow excitation function can tell about the quark-gluon plasma

摘要

Recent STAR data from the RHIC beam energy scan (BES) show that the slope of the net-proton directed flow v(1) at midrapidity changes sign twice within the collision energy range 7.7-39 GeV. To investigate this phenomenon, we study the collision energy dependence of v(1) utilizing a hybrid model which combines Boltzmann transport and hydrodynamic approach. Calculations with dynamically evolved initial and final state show no qualitative difference between an equation of state with a cross-over and one with a first-order phase transition, in contrast to earlier pure fluid predictions. Our analysis of the elliptic flow v(2) shows that pre-equilibrium transport dynamics are partially compensating for the diminishing contribution to the elliptic flow from the hydrodynamical phase at lower energies, which leads to a qualitative agreement with STAR BES results in midcentral collisions. No compensation from transport is found for integrated v(3), which decreases from approximate to 0.02 at root S-NN = 27 GeV to approximate to 0.005 at root S-NN = 7.7 GeV in midcentral collisions. (C) 2014 Elsevier B.V. All rights reserved.