Probing neutron-proton dynamics by pions

TL;DR

This study investigates neutron-proton dynamics and the high-density symmetry energy in nuclear reactions by analyzing pion production in neutron-rich nuclei collisions, revealing a direct link between delta resonance ratios and pion ratios.

Contribution

The paper introduces a combined AMD and JAM approach to connect neutron-proton dynamics with pion production, highlighting the role of delta resonances in high-density nuclear matter.

Findings

Delta production ratio matches neutron-proton squared ratio (N/Z)^2

Pion ratio reflects the delta resonance ratio with final-stage modifications

High-density symmetry energy influences nucleon dynamics and pion production

Abstract

In order to investigate the nuclear symmetry energy at high density, we study the pion production in central collisions of neutron-rich nuclei ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ at 300 MeV/nucleon using a new approach by combining the antisymmetrized molecular dynamics (AMD) and a hadronic cascade model (JAM). The dynamics of neutrons and protons is solved by AMD, and then pions and $\Delta$ resonances in the reaction process are handled by JAM. We see the mechanism how the $\Delta$ 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 $\Delta^-/\Delta^{++}$ 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 $\Delta$ production ratio, and therefore $(N/Z)^2$, are directly reflected in the $\pi^-/\pi^+$ ratio, with modification in the final stage of the reaction.