碰撞参数的混杂效应及其对观测量的影响

基于改进型量子分子动力学（ImQMD）模型，讨论了中低能区重离子碰撞实验上利用多重数定义的中心碰撞和理论上利用固定碰撞参数定义的中心碰撞的差别及其影响。在束流能量为50 MeV/u时，发现两种方法得到的碎块电荷多重数分布、碎块的平均动能的结果的最大偏差约有50%。两种方法对于同位旋敏感观测量--并合不变的中子-质子产额比的影响在5%~15%之间。研究表明，利用中低能重离子碰撞精确获取核物质状态方程或者介质中核子-核子散射截面时，必须考虑碰撞参数混杂的效应。

Impact Parameter Smearing and Its Influence on Heavy Ion Collision Observables

Based on the Improved Quantum Molecular Dynamics model (ImQMD), the difference of central collision in experiment and theory for low-intermediate energy heavy ion collisions is investigated. By analyzing the two kinds of central collisions, we find there is 50% difference on the charge distribution, and also on its average kinetic energy for heavier fragments, especially at the beam energy of 50 MeV/u. While, for the coalescence invariant neutron proton yield ratio, the calculations show there are about 5% ~ 15% difference. Our results evidence that the impact parameter smearing effects must be considered for tightly constraining the nuclear equation of state and the in-medium nucleon-nucleon cross sections by heavy ion collision.