中能重离子碰撞中π介子产生的介质效应和高密对称能

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.

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。

强相互作用物质的性质通常由状态方程描述，它在核物理和核天体物理中都有着基本的重要性。特别是其在高密或者高同位旋不对称度的极端条件下的行为以及引起了人们的广泛关注。重离子碰撞提供了一种研究极端条件下核物质性质的独特途径。重离子碰撞中的很多观测量或现象都与核物质性质密切相关。例如中能重离子碰撞中的荷电π介子比就被认为是高密(约2倍饱和密度)对称能的敏感探针。本项目中围绕着强相互作用物质和重离子碰撞，我们主要开展了如下研究：.1）在Skyrme能量密度泛函框架下，通过原子核基态与集体激发态性质的贝叶斯分析，约束了核物质对称能与不可压缩系数，以及核物质中的核子有效质量。.2）发展了一套新的格点哈密顿量方法用于求解Boltzmann-Uehling-Uhlenbeck（BUU) 。得益于GPU并行计算，该方法达到了较高精度。基于这一新工具，我们研究了原子核的巨偶极共振（GDR）宽度， 发现Pb208的GDR宽度对介质中的核子-核子弹性散射截面非常敏感。对比GDR宽度的实验数据要求核子-核子弹性散射截面在介质中有大的衰减因子。.3）基于改进的相对论的Vlasov-Uehling-Uhlenbeck (RVUU) 模型，我们研究了质心能量2.4GeV的Au+Au碰撞中的荷电π介子的产生，发现核子-核子非弹性散射截面的介质修正对于描述实验测得的π介子的产额与能谱至关重要。.4）基于夸克准粒子模型，研究了夸克物质和夸克星的性质。.这些工作加深了对强相互作用物质的认识，将在致密星体与重离子碰撞的研究中发挥重要作用。