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Yong-Jia Wang and Qing-Feng Li, Application of microscopic transport model in the study of nuclear equation of state from heavy ion collisions at intermediate energies, Front. Phys. 15(4), 44302 (2020), arXiv: 2004.11737 (19 pages)
利用微观输运模型和中能重离子碰撞研究核状态方程
核物质状态方程指的是核子的能量与核物质密度、温度以及同位旋不对称度之间的热力学关系。核状态方程一直以来都是核物理、天体物理领域关心的重要课题,因为它的信息对研究原子核结构、核碰撞、中子星的结构及其并合过程等都十分重要。重离子碰撞是在实验室中制造高密核物质的唯一途径,但是产生的致密物质存在的时间十分短暂,大约10-23~10-22 秒,目前还不能在实验中直接测量核物质状态方程。通常利用把唯象势作为输入量的输运模型来模拟重离子碰撞过程,通过与末态实验观测数据的比较来获取核状态方程的信息。极端相对论量子分子动力学(UrQMD)模型是一个被广泛使用的微观输运模型,可以描述入射动能从每核子几十MeV(费米能量)到质心系能量为几个 TeV(LHC能区)的重离子碰撞。通过对UrQMD模型的平均场势能项、碰撞项以及碎片构造方式等的改进,该模型可以较好的再现FOPI实验组给出的轻带电粒子的集体流和核阻止本领的实验数据。本文着重介绍了基于UrQMD模型研究核状态方程和对称能的最新结果,并讨论了从输运模型和重离子碰撞实验中提取核状态方程的新机遇和新挑战。
The equation of state (EOS) of nuclear matter, i.e., the thermodynamic relationship between the binding energy per nucleon, temperature, density, as well as the isospin asymmetry, has been a hot topic in nuclear physics and astrophysics for a long time. The knowledge of the nuclear EOS is essential for studying the properties of nuclei, the structure of neutron stars, the dynamics of heavy ion collision (HIC), as well as neutron star mergers. HIC offers a unique way to create nuclear matter with high density and isospin asymmetry in terrestrial laboratory, but the formed dense nuclear matter exists only for a very short period, one cannot measure the nuclear EOS directly in experiments. Practically, transport models which often incorporate phenomenological potentials as an input are utilized to deduce the EOS from the comparison with the observables measured in laboratory. The ultrarelativistic quantum molecular dynamics (UrQMD) model has been widely employed for investigating HIC from the Fermi energy (40 MeV per nucleon) up to the CERN Large Hadron Collider energies (TeV). With further improvement in the nuclear mean-field potential term, the collision term, and the cluster recognition term of the UrQMD model, the newly measured collective flow and nuclear stopping data of light charged particles by the FOPI Collaboration can be reproduced. In this article we highlight our recent results on the studies of the nuclear EOS and the nuclear symmetry energy with the UrQMD model. New opportunities and challenges in the extraction of the nuclear EOS from transport models and HIC experiments are discussed.
Keywords nuclear equation of state, symmetry energy, heavy ion collision, transport model
Contents
1 Introduction
2 Model description and observables
2.1 Mean field potential
2.2 The in-medium nucleon-nucleon cross section
2.3 Observables
3 Influence of the in-medium nucleon-nucleon cross section on observables
4 Determination of the nuclear incompressibility
5 Constraints on the density-dependent nuclear symmetry energy
5.1 Result from 3H/3He yield ratio
5.2 Result from the elliptic flow ratio between neutrons and hydrogen isotopes
6 Summary and outlook
Acknowledgements
References
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