中文摘要：

利用最近更新的极端相对论量子分子动力学（UrQMD）模型研究了对称能对重离子碰撞中的直接流平衡能量（Ebal）的影响。以197Au＋197Au半中心（约化碰撞参数为b0=0.15-0.4）碰撞为例,来研究不同密度依赖的对称势能对Ebal的影响。改进之后的模型给出的直接流斜率的激发函数能够很好的与最新的实验数据符合,通过50万次事件的模拟及利用三点直线拟合的方法,给出了非常精确的平衡能量。研究结果表明,直接流的Ebal依赖于粒子的种类,不同的密度依赖的对称能对质子直接流的Ebal影响很小（小于1 MeV）,而对质子数为1的粒子（包括质子、氘核、氚核）及中子的直接流的Ebal影响较为明显,大约有几个MeV。

英文摘要：

The influence of the density-dependent symmetry energy on the balance energy（Ebal） of directed flow from 197Au＋197Au collisions is studied,using the updated version of the ultra-relativistic quantum molecular dynamics model（UrQMD）,especially adapted to low-energy heavy-ion collisions（HICs）.The improved version can reproduce the recent experimental data very well,more than 500 thousand events were calculated,and the balance energy can be fixed accurately.The results show that the uncertainty in the density dependence of the symmetry potential energy causes changes of balance energy of the order of several MeV,depending on the type of particle considered.The balance energy of Z=1 particles and neutrons from HICs are particularly sensitive to the density dependent nuclear symmetry energy,while balance energy of protons is almost not influenced by various density dependent nuclear symmetry energy in this system.