Probing neutron-proton dynamics by pions

In order to investigate the nuclear symmetry energy at high density, we study the pion production in central collisions of neutron-rich nuclei ¹³²Sn+¹²⁴Sn at 300 MeV/nucleon using a new approach that combines antisymmetrized molecular dynamics (AMD) and a hadronic cascade model (JAM). The dynamics of neutrons and protons is solved by AMD, and then pions and Δ resonances in the reaction process are handled by JAM. We see the mechanism by which the Δ resonance and pions are produced, reflecting the dynamics of neutrons and protons. We also investigate the impacts of cluster correlations as well as of the high-density symmetry energy on the nucleon dynamics and consequently on the pion ratio. We find that the Δ^-/Δ⁺⁺ production ratio agrees very well with the neutron-proton squared ratio (N/Z)² in the high-density and high-momentum region. We show quantitatively that the Δ production ratio, and therefore (N/Z)², are directly reflected in the π^-/π⁺ ratio, with modification in the final stage of the reaction.