Constraining the nuclear symmetry energy with Fermi-energy heavy ion collisions

Abstract

Heavy ion reactions provide a unique opportunity to unveil the Equation of State (EoS) of baryonic matter in a large density domain. However, to get quantitative constraints it is crucial to employ observables that are as insensitive as possible to final state interaction, and at the same time robustly predicted by transport models with limited model dependence. In this work, we compare for the first time BUU transport calculations to the impact parameter dependence of the isospin transport ratio deduced from INDRA-FAZIA data [1], with a model independent evaluation of the impact parameter. Using different state-of-the-art nuclear functionals, provided both by fits of ab initio calculations and by phenomenological approaches, a confidence region for the symmetry energy is extracted. A consistent study of the time dependence of the baryonic density and of the isospin current density allows a precise determination of the density region significantly probed by the experiment, with the definition of confidence regions in the symmetry energy vs density plane. A symmetry energy 𝑆 = (29.0 ± 0.7) MeV is obtained for the most significant density 𝜌∕𝜌0 = 1.01. The obtained symmetry energy constraint can be used to inform Bayesian inference of the neutron star EoS.