Shedding light on the pion production in heavy-ion collisions and application into the neutron star matter properties
Within the framework of the quantum molecular dynamics transport model, the pion production and constraint of the high-density symmetry energy in heavy-ion collisions near threshold energy have been thoroughly investigated. The energy conservation in the decay of resonances and reabsorption of pions as well as in the inelastic nucleon-nucleon and nucleon-resonance collisions are taken into account. The isospin diffusion in the low-density region (0.2$ρ_{0}$ - 0.8$ρ_{0}$) and high-density region (1.2$ρ_{0}$ - 1.8$ρ_{0}$) is investigated by analyzing the spectra of neutron/proton and $π^{-}/π^{+}$ ratios in the isotopic reactions of $^{132}$Sn + $^{124}$Sn and $^{108}$Sn + $^{112}$Sn at the incident energy of 270 MeV/nucleon, in which the symmetry energy manifests the opposite effect in the different density domain. The controversial conclusion of the $π^{-}/π^{+}$ ratio for constraining the high-density symmetry energy by different transport models with the FOPI data has been clarified. A soft symmetry energy with the slope parameter of $L(ρ_{0}) = 42\pm 25$ MeV by using the standard error analysis within the range of $1σ$ is obtained by analyzing the experimental data from the S$π$RIT collaboration. The neutron stars with the maximal mass of 2 $M_{\odot}$ and radius of 11-13 km are obtained with the constrained symmetry energy.