中能重离子碰撞中π介子产生的介质效应和高密对称能

核物质对称能决定着核物质状态方程的同位旋相关性，它在核结构、核反应和核天体领域都有着重要意义。过去的几十年里，人们通过不同的理论模型，实验探针乃至天文观测量，探索了对称能的密度相关性。目前，人们对饱和密度附近的对称能已经有了相对较清楚的认，但是对称能的高密行为仍有很大的不确定性。中能重离子碰撞中产生的荷电π介子比值被认为是高密对称能的理想探针。虽然已经有很多工作利用不同的输运模型试图从这一实验探针中提取高密对称能，但是得到结论却很不一致。因此，更深入的理论研究尤为重要。本项目将发展相对论的Vlasov-Uehling-Uhlenbeck(RVUU)模型研究介质效应，如核子势的动量相关性，Δ共振态的对称势，介质中Δ的产生截面和衰变宽度等对π介子产生的影响。随后，将基于改进后的RVUU模型，从中能重离子碰撞中的π介子相关的实验探针中提取对称能的高密行为。

Nuclear symmetry energy, characterizing the isospin-dependence of nuclear equation of state, is of fundamental importance in both nuclear physics and astrophysics. While the symmetry energy at or around the saturation density has been relatively well constrained by analyses of various experimental probes and astrophysical observables as well as predictions of microscopic many-body theories, its high-density behavior remains very uncertain and has attracted much attention. Heavy-ion collisions of neutron-rich nuclei provides a unique approach to study the high-density behavior of the symmetry energy in terrestrial laboratories. In particular, the charged pion ratio in intermediate-energy heavy-ion collisions has been proposed as a promising probe of high-density (about two times saturation density) symmetry energy. And many attempts using various transport models have been made to extract high-density behavior of the symmetry energy from experimental data on charged pion ratio. However, these studies have led to quite different conclusions, and there is still no qualitative consensus on this issue. A more in-depth understanding of pion production, such as the threshold effects, impact of energy conservation, and pion in-medium effects, is therefore important. This project will employ a relativistic Vlasov-Uehling-Uhlenbeck (RVUU) model to study medium effects on pion production in intermediate-energy heavy-ion collisions, including effects originated from the momentum dependence of nucleon potential, symmetry potential of the Δ resonance , and in-medium Δ production cross section and decay width. Based on the improved RVUU model with the inclusion of these medium effects, the high-density behavior of nuclear symmetry energy will be further extracted from experimental measurements of charged pion ratio or other pion observables from intermediate-energy heavy-ion collisions.