Analysis On The Dynamical Mechanism For Elliptic Flow Production In Heavy Ion Collisions At Intermediate Energies

Nuclear symmetry energy has always been a hotspot in the field of nuclear physics,and elliptic flow has attracted much attention as a sensitive observable to explore the nuclear symmetry energy.The elliptic flow characterizes the extrusion of the nuclei in the vertical direction after the collision of two nuclei,and carries information about the high-density area formed during the collision.The transport model is one of the important tools for studying heavy ion collisions.It mainly includes two parts in the dynamic progress,nucleon-nucleon collision and mean field.This work is to explore the influence of these two parts on the formation and evolution of elliptical flow.Both nucleon-nucleon collisions and mean field contribute to the elliptic flow signal.In the simulation,we can distinguish their contributions and study how they develop as a function of time.It is mainly achieved by recording the elliptic flow of nuclei before and after each collision and before and after the mean field.This work is based on the Ultra-Relativistic Quantum Molecular Dynamics(Ur QMD)model to track the nucleons in the central rapidity zone(|y₀|?0.1)of the final state of the reaction,and explore the evolution of these nucleons over time.By recording these nucleons before and after the mean field and nucleon-nucleon collisions respectively,we can explore the contribution of these two parts.The results show that for Au+Au collision with an incident energy of 0.4 Ge V per nucleus and impact parameter of 5 fm,the contribution of the mean field to the v₂ is about 30%,and the contribution of the collision to it accounts for 70%.It is also found that when Pauli blocking is not considered in the collision term,the value given is 10%larger than that when Pauli blocking is considered.At the incident energy of 0.4 Ge V/nucleon,the elliptic flow tends to rise first,then descend rapidly and finally become stationary.At the first stage,the positive elliptic flow was generated due to the attraction of the average potential field.As the reaction proceeds,the two nuclei collide and a negative elliptical flow is produced by the interaction of the potential field and the collision.At the final stage,when the collision is over,the nucleon is only affected by the mean field and the change trend is relatively stable.