Preequilibrium particle emissions and in-medium effects on the pion production in heavy-ion collisions

Within the framework of the Lanzhou quantum molecular dynamics (LQMD) transport model, pion dynamics in heavy-ion collisions near threshold energies and the emission of preequilibrium particles (nucleons and light complex fragments) have been investigated. A density, momentum and isospin-dependent pion-nucleon potential based on the Δ-hole model is implemented in the transport approach, which slightly leads to the increase of the π^-/π⁺ ratio, but reduces the total pion yields. It is found that a bump structure of the π^-/π⁺ ratio in the kinetic energy spectra appears at the pion energy close to the Δ(1232) resonance region. The yield ratios of neutrons to protons from the squeeze-out particles perpendicular to the reaction plane are sensitive to the stiffness of nuclear symmetry energy, in particular at the high-momentum (kinetic energy) tails. (orig.)