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

TL;DR

This study investigates the energy dependence of collective flow in heavy-ion collisions using a hybrid model, revealing insights into the quark-gluon plasma and the nature of the phase transition.

Contribution

It demonstrates that a hybrid model with dynamic initial and final states shows no qualitative difference between crossover and first-order phase transitions, contrasting earlier pure fluid predictions.

Findings

The directed flow slope dv1/dy changes sign twice within 7.7-39 GeV.

Pre-equilibrium transport partially compensates for reduced elliptic flow at lower energies.

Integrated v3 decreases significantly from 27 GeV to 7.7 GeV.

Abstract

Recent STAR data from the RHIC beam energy scan (BES) show that the midrapidity slope dv1/dy of the directed flow v1 of net-protons changes sign twice within the collision energy range 7.7 - 39 GeV. To investigate this phenomenon, we study the collision energy dependence of v1 utilizing a Boltzmann + hydrodynamics hybrid model. 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. Furthermore, our analysis of the elliptic flow v2 shows that pre-equilibrium transport dynamics are partially compensating for the diminished elliptic flow production in 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 in our model for integrated v3, which decreases from approx 0.02 at sqrt(sNN) = 27 GeV to approx 0.005 at sqrt(sNN) = 7.7 GeV in midcentral collisions.