# 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 how non-equilibrium effects influence system evolution and final observables, emphasizing the importance of initial pre-equilibrium flow.

## Contribution

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

## Key findings

- Non-equilibrium effects cause large fluctuations in collective properties.
- Ensemble-averaged observables align closely with hydrodynamical results.
- Initial pre-equilibrium flow significantly affects final state 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.