Collective Dynamics Studying On UU Collisions At CSR Energy

LanZhou HIRFL-CSR is one of the most important science engineerning for the 95 science plan in china. It will provide us a lot of chance to gain international advanced results and also have good experimental condition in the nuclear physics and heavy ion colliding physics.It is important and necessary for the experiment of nucleus-nucleus collisions to use some well-know models simulating and studying it. In this article, we use a Relativistic Transport Model (ART1.0) to study the UU collision with a beam energy of 0.52GeV/nucleon for the external target experiment in CSR. Uranium is the most deformed stable nucleus. For ²³⁸U, the ratio of the long-axis over short-axis is as large as 1.3. Because of the deformation, UU collisions at the same beam energy and impact parameter but different orientations are expected to form dense matter with different compressions and lifetimes. In particular by selecting the tip-tip UU collision which are defined as the head on collisions with long-axis on long-axis, one may reach high multiplicity, extend the reaction duration and both energy and particle density in high energy nuclear collisions. This is a powerful tool for studying the physics of large compression, high-baryon density, possible phase transition and medium effect for high density nuclear matter.By studying the time evolvement of center local baryon and energy density in the two extreme UU collisions with different EOS (Equation of State), we find that the reaction duration in tip-tip UU collision is longer than body-body, but the maximal baryon and energy density is nearly same. By contact with the evolvement of the particles multiplicity, a logical answer is presented that the whole collision is persisted step by step. The particles in the early stage can't escape from the overlap area because of the strong attractive mean-field and interact with the latter participant, which induce the energy aggradation. At the same time the particles with higher transverse momentum in the early stage go away from the reaction area, which will weaken the deposit energy.The final pions are producted directly by the interaction of nucleons-nucleons or by the decay of resonance baryonsâ–³and N^*. The final kaons are mostly producted by the secondary interaction of particles.The nucleus stopping power is very strong in CSR, thus the local thermalization of high density matter will be discovered very possibly in center tip-tip UU collisions. By using the exponential function and the thermal-radial flow model to fit the transverse mass and momentum spectra respectively, we find that in center UU collisions the thermal temperature is about 55MeV,the mean transverse flow profile is about 0.16c and the radial flow velocity is about 0.2c.The transverse flow and directed flow of final particles in the reaction plane have a evident wiggle slope in mid-rapidity and a obvious difference for the pions and nucleons as a result of the shadowing of spectators through rescatterings and reabsorptions. There is a transition from pion transverse flow to antiflow at an impact parameter of about 2fm.Flow is advanced mainly in the early stage and saturates in the expand phase of the system. There is a strong dependence between the particles flow and central-ity,rapidity or transverse momentum. At b = 0, directed flow vanishes but elliptic flow reaches the maximum in body-body UU collision,about 12%,as a result of the initial coordinate space asymmetry. Because of the squeeze-out effect,elliptic flow is always out-of-plane. Elliptic flow in low transverse momentum is according with hydrodynamics. Recur to the obvious different nucleons elliptic flow in center tip-tip and body-body UU collisions,one can selects and distinguishes those interested events for studying the properties of high density matter.