High-density symmetry energy and pion production in heavy-ion collisions

The density dependence of the nuclear symmetry energy is a valuable information for understanding neutron rich systems such as the nuclear structure, heavy-ion collisions, neutron stars and their mergers. Especially, heavy-ion collisions are believed to be useful to investigate the symmetry energy at high densities. Since the neutron-proton ratio in the high-density region is not a direct observable, the pion ratio has been proposed to be a good probe to constrain the high-density behavior of the symmetry energy.

 

At present, some theoretical studies have been performed by different transport models to investigate the sensitivity of pion observables in heavy-ion collisions. However, some of these results are contradicting to each other even qualitatively. Therefore, it is necessary to understand the relation between the pion ratio and the nucleon dynamics to deduce the information on the symmetry energy. In the experimental studies, Sn+Sn collisions have been performed at RIBF/RIKEN.

 

In this presentation, we report the results for the pion production in central collisions of neutron-rich nuclei using our transport model that combines antisymmetrized molecular dynamics (AMD) and a hadronic cascade model (JAM). In our calculation, we found the mechanism how the delta resonance and pions are produced reflecting the dynamics of neutrons and protons.