Traces of non-equilibrium dynamics in relativistic heavy-ion collisions

TL;DR

This paper compares non-equilibrium transport and viscous hydrodynamical models in heavy-ion collisions, revealing significant fluctuations due to non-equilibrium effects and emphasizing the importance of initial pre-equilibrium flow.

Contribution

It introduces a detailed comparison between non-equilibrium transport and hydrodynamical models, highlighting the impact of non-equilibrium phenomena on collision dynamics.

Findings

Large fluctuations in collective properties from transport models

Ensemble averages align with hydrodynamical results

Pre-equilibrium flow significantly affects final observables

Abstract

The impact of non-equilibrium effects on the dynamics of heavy-ion collisions is investigated by comparing a non-equilibrium transport approach, the Parton-Hadron-String-Dynamics (PHSD), to a 2D+1 viscous hydrodynamical model, which is based on the assumption of local equilibrium and conservation laws. Starting the hydrodynamical model from the same non-equilibrium initial condition as in the PHSD, using an equivalent lQCD Equation-of-State (EoS), the same transport coefficients, i.e. shear viscosity $\eta$ and the bulk viscosity $\zeta$ in the hydrodynamical model, we compare the time evolution of the system in terms of energy density, Fourier transformed energy density, spatial and momentum eccentricities and ellipticity in order to quantify the traces of non-equilibrium phenomena. In addition, we also investigate the role of initial pre-equilibrium flow on the hydrodynamical evolution and demonstrate its importance for final state observables. We find that due to non-equilibrium effects, the event-by-event transport calculations show large fluctuations in the collective properties, while ensemble averaged observables are close to the hydrodynamical results.