基于输运模型对核子有效质量劈裂、高密对称能及相关的模型依赖问题的研究

本项目基于微观输运模型---极端相对论量子分子动力学(UrQMD)模型，对有可能影响核子有效质量劈裂和高密对称能实验观测量（如中子质子双比、π介子产额比和中子质子椭圆流之比）的数个因素做细致全面地分析，这些因素包括初始化时中子质子的密度分布、平均场势修正（包括π介子和Δ共振态）、Δ共振态产生道的介质效应及末态碎片构建等。通过与实验数据的比较对模型进行优化，在此基础上找出受模型参数影响更小、敏感度更高的观测量或者实验探测窗口，为将来开展的有关实验提供指导。本项目还将在UrQMD模型中引入其它输运模型（如ImQMD，IBUU，LQMD，BLE等）中通常处理初始化、平均场、碰撞项及碎片构建部分的方法和手段，通过仔细比对计算的结果，找出在提取核子有效质量劈裂和高密对称能信息时出现的模型及观测量依赖的原因；结合最新的相关实验进展，采用多观测量约束，自洽地提取出核子有效质量劈裂和高密对称能的信息。

In this project, based on a microscopic transport model – the ultra-relativistic quantum molecular dynamics (UrQMD) model, we will make a detailed study on some issues, including the initial neutron and proton density distribution, mean field treatments including π and Δ resonance, medium modification of the Δ production channels, and cluster recognition criteria, which may influence experimental observables (e.g. the neutron-proton double ratio, π-/π+ yield ratio, and neutron-proton elliptic flow ratio) sensitive to the neutron-proton effective mass splitting and nuclear symmetry energy. After optimizing our model through a careful comparison of calculations with corresponding experimental data, observables and/or detection conditions with high sensitivity and reliability are hoped to be obtained for the guidance of forthcoming related experiments. Meanwhile, various treatments of initialization, mean field, collision term, and cluster recognition in other transport models (e.g ImQMD, IBUU, LQMD, BLE) will be examined in the UrQMD framework. After the comparison of calculation results under different model assumptions, the reason why some results are model dependent will be clear up. Further, following up the recent progress in experiments, more self-consistent information about both the nucleon effective mass splitting and the symmetry energy especially at high densities will be extracted.