Influence Of Symmetry Energy On Disappearance Of Directed Flow In Heavy Ion Collisions

The knowledge of equation of state of nuclear matter at high density andtemperature is very important in nuclear physics and astrophysics, and it is one of hottopics of nuclear physics.In this work, firstly, we give an introduction of the background of nuclearsymmetry energy and collective flow in heavy ion collisions. We further brieflyintroduce two main theoretical models, Boltzmann- Uehling- Uehlenbeck（BUU） andquantum molecular dynamics（QMD） model. Secondly, we update the ultra-relativisticquantum molecular dynamics model （UrQMD）, the main improvements are to addmomentum dependence mean field, to modify Pauli-blocking, and to change thedensity and momentum dependent in-medium nucleon-nucleon elastic cross section.Thirdly, we reproduce the experimental observables, such as the yield, the collectiveflow and the balance energy (E_bal). Then, within the updated version of UrQMDmodel, we study the influence of symmetry energy on the disappearance of directedflow.Many different collision systems are chosen for investigating the influence of thesymmetry energy on the E_balof directed flow. The results show that the uncertainty inthe density dependence of the symmetry potential energy causes changes of E_balof theorder of several MeV, depending on the type of particle considered. The E_balof Z=1particles and neutrons from HICs are particularly sensitive to the density dependenceof the symmetry energy, while E_balof protons is not. So it is very useful forforthcoming experimenters to investigate symmetry energy by neutron detection.