Azimuthal anisotropies for Au+Au collisions in the parton-hadron transient energy range

TL;DR

This paper investigates azimuthal anisotropies in heavy-ion collisions across various energies using the PHSD model, highlighting the increasing role of quark-gluon degrees of freedom in the early collision phase.

Contribution

It demonstrates that the PHSD approach successfully describes the energy dependence of flow observables and reveals the significance of partonic effects at higher energies.

Findings

PHSD reproduces the energy dependence of elliptic flow $v_2$.

Higher-order flow harmonics show increasing deviations between partonic and hadronic models with energy.

The ratio $v_4/(v_2)^2$ varies with energy, centrality, and transverse momentum, indicating underlying dynamics.

Abstract

The azimuthal anisotropies of the collective transverse flow of charged hadrons are investigated in a wide range of heavy-ion collision energies within the microscopic parton-hadron-string dynamics (PHSD) transport approach which incorporates explicit partonic degrees of freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation of state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow $v_2$ of charged hadrons with collision energy is successfully described in terms of the PHSD approach. The PHSD scaling properties of various collective observables are confronted with experimental data as well as with hydrodynamic predictions. The analysis of higher-order harmonics $v_3$ and $v_4$ in the azimuthal angular distribution shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing collision energy. This demonstrates that the excitation functions of azimuthal anisotropies reflect the increasing role of quark-gluon degrees of freedom in the early phase of relativistic heavy-ion collisions. Furthermore, the specific variation of the ratio $v_4/(v_2)^2$ with respect to bombarding energy, centrality and transverse momentum is found to provide valuable information on the underlying dynamics.