Nuclear equation-of-state at high density and multi-messenger astronomy: contribution of heavy-ion collisions

Abstract

In the past decades, heavy-ion collisions (HIC) at intermediate energies have allowed to probe the nuclear equation-of-state (EoS) of both symmetric and asymmetric nuclear matter over a broad range of densities. In particular, flow has proven to be a powerful observable. Combining the symmetry energy and the symmetric nuclear matter constraints of the EoS from HIC allowed to predict a density dependence of the pressure in a neutron star, up to about 2.5 times saturation density ($n_{sat}$), which agrees with recent astronomical measurements deduced from gravitational waves and pulsar observations. So far, the accuracy from HIC expectations is comparable to the latter up to 1.5 $n_{sat}$. In these studies, a fundamental aspect is the determination of the profile of densities that are probed by experimental observables used to constrain the EoS. In the near future, new experiments like ASY-EOS performed at higher incident energy and with better accuracy will push further the frontier of the knowledge of the symmetry energy at higher density. These efforts cannot be conclusive without a reliable uncertainty determination, which is related to the reliability of transport model dependencies. Improvements and breakthroughs in transport model simulations and nuclear theory are therefore expected in a joint effort towards HIC contributions to the field of neutron-star physics, including the contribution of strangeness and of the QCD phase transition.