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

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 $${\sqrt{s}_\mathrm{NN}}$$ s 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.

Enhanced dilepton emission from a phase transition in dense matter

“…Though the relaxation time is predicted to be smaller than the interpenetration time [28], the emission of dileptons in the very early stages of collisions is often not considered because of this uncertainty. On the other hand, a recent study has shown that shortly after interpenetration, the systems described do reach a state of local isotropy and equilibrium, and the assumption of a near-hydro T μ ν is justified [64]. In addition to extracting the local density and temperature, the coarse-graining procedure allows the local densities of π and ρ mesons and Δ baryons to be determined, which can be used to check whether a system is in local chemical equilibrium.…”