Traces of nonequilibrium dynamics in relativistic heavy-ion collisions
The impact of nonequilibrium effects on the dynamics of heavy-ion collisions is investigated by comparing a nonequilibrium 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 nonequilibrium initial condition as in the PHSD, using an equivalent lQCD equation of state (EoS), the same transport coefficients, i.e., shear viscosity η and the bulk viscosity ζ 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 to quantify the traces of nonequilibrium 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 because of nonequilibrium effects, the event-by-event transport calculations show large fluctuations in the collective properties, while ensemble averaged observables are close to the hydrodynamical results.