The applicability of hydrodynamics in heavy ion collisions at $\sqrt{s_{\rm NN}}$= 2.4-7.7 GeV

TL;DR

This paper investigates the applicability of hydrodynamics in low to intermediate energy heavy-ion collisions by analyzing the system's degree of equilibration using a hadronic transport model and coarse-graining, with implications for hybrid modeling approaches.

Contribution

It provides a detailed assessment of local equilibration conditions in heavy-ion collisions at 2.4-7.7 GeV, highlighting the timescales and event-by-event variability relevant for hydrodynamic modeling.

Findings

Hydrodynamic conditions are often met in average evolution but rarely event-by-event.

Local equilibration persists for several femtometers per c during the evolution.

Implications for hybrid models at low energies in upcoming experiments.

Abstract

To assess the degree of equilibration of the matter created in heavy-ion reactions at low to intermediate beam energies, a hadronic transport approach (SMASH) is employed. By using a coarse-graining method, we compute the energy momentum tensor of the system at fixed time steps and evaluate the degree of isotropy of the diagonal terms and the relative magnitude of the off-diagonal terms. This study focuses mostly on Au+Au collisions in the energy range $\sqrt{s_{\rm NN}}$ = 2.4-7.7 GeV, but central collisions of lighter ions like C+C, Ar+KCl and Ag+Ag are considered as well. We find that the conditions concerning local equilibration for a hydrodynamic description are reasonably satisfied in a large portion of the system for a significant amount of time (several fm/c) when considering the average evolution of many events, yet they are rarely fulfilled on an event by event basis. This is relevant for the application of hybrid approaches at low beam energies as they are or will be reached by the HADES experiment at GSI, the future CBM experiment at FAIR as well as the beam energy scan program at RHIC.