中能重离子碰撞中π介子产生的核介质效应和集体流研究

In-medium effect and collective flow of pion production in intermediate energy heavy-ion collisions

本文基于量子分子动力学输运模型(LQMD)研究了中能重离子碰撞中π介子产生机制,包括多重性时间演化、快度分布、横向动量分布、集体流等物理量.以反应系统^(40)Ca+^(40)Ca,^(90)Zr+^(90)Zr,^(96)Zr+^(96)Zr,^(96)Ru+^(96)Ru和^(197)Au+^(197)Au为例,我们研究了Δ-核子相互作用势、核介质中散射截面、π介子-核子相互作用势和散射截面对π介子产生的影响.结果给出同位旋依赖的Δ-核子作用势会增加π^(−)/π^(+)的比值,考虑核介质效应的Δ-核子散射截面会压低π介子的产生,特别是在中心快度和低动量区域.基于π介子与原子核散射的实验数据,提取了π势并分析了快度分布、横向动量谱、集体流.π势会压低高横向动量和中心快度区域π介子产生,特别是π^(−)产生.π介子与核子散射吸收过程会导致反常流现象,特别是π^(+)直接流.三角流与直接流具有相似的分布形式,四阶流与椭圆流分布相似.

The production mechanism of pion in intermediate energy heavy-ion collisions has been studied by LQMD(Lanzhou Quantum Molecular Dynamics)model.The time evolution of multiplicity,rapidity distribution,transverse momentum spectra,collective flows,etc.,is calculated in collisions of ^(40)Ca+^(40)Ca,^(90)Zr+^(90)Zr,^(96)Zr+^(96)Zr,^(96)Ru+^(96)Ru and ^(197)Au+^(197)Au.TheΔ-nucleon interaction potential and scattering,π-nucleon potential and scattering and stiffness of symmetry energy are to be investigated systematically.It is found that theπ^(−)/π^(+)ratio is influenced by the isospin-dependentΔ-nucleon potential and the pion production is suppressed by the in-mediumΔ-nucleon scattering,in particular in the region of midrapidity and low-momentum of pion production.The transverse momentum spectra,rapidity distribution,collective flows andπ^(−)/π^(+)ratio are influenced by the isospin-dependentπ-nucleon potential,in particular in the mid-rapidity and high-momentum region.The antiflow ofπ^(+)production is caused by the shadowing effect of participant nucleons.The triangular flow(Vx_(3))is similar to the directed flow(Vx_(1)),but the magnitude of quadrangular flow(V4)is very small for the pion production.