【作者】 吴科军；

【导师】 刘峰；

【作者基本信息】 华中师范大学 ， 粒子物理与原子核物理， 2007， 硕士

【摘要】 兰州重离子加速器-冷却储存环（HIRFL-CSR）是我国九五国家重大科学工程，它将为开展核状态方程和强作用物质性质随物质密度变化、重子共振态、介子阈下产生机制、手征对称破缺恢复等核物理问题的研究提供良好的机遇。利用物理模型去模拟核核碰撞实验过程，能为实验提供一定的参考和预测。本文利用相对论输运模型ART模拟并研究了CSR外靶²³⁸U⁷²⁺+²³⁸U⁷²⁺实验（E_b=0.52GeV／u）。U核是最大的形变核（形变因子为0.27，长短轴之比为1.3）!由于它的非球对称性，当UU沿不同方位碰撞时，将导致碰撞过程中形成的中央高密物质不同的压缩率和寿命。理论上预言，特别是当发生对心头对头UU碰撞时，高密物质密度将比体对体碰撞时要高得多，也比其他球对称核要高。这为研究高密度物质状态下可能存在的相变和介质效应等是非常有利的。中心重子密度和能量密度的时间演化表明，头对头UU碰撞高密相维持时间明显长于体对体碰撞，但是它们所能达到的最大中心重子密度和能量密度近似相同，结合粒子多重数的时间演化，一个合理的解释是：早先在核核碰撞重叠区发生碰撞产生的粒子，受到强烈的平均场作用，并不能很快飞离中心区域，它们会与后期进入重叠区的粒子发生反应，导致中心区域能量密度累积，同时部分早期产生的高能粒子会飞离中心区域，使沉积能量有所削弱。该能量下核阻止本领很强，最有可能在对心头对头UU碰撞里出现局域热化。拟合横质量谱和横动量谱的结果显示，对心碰撞下，热化温度大约为55MeV，对应的平均横向流速为0.16c，冻结时径向流速为0.2c。末态粒子侧向流和直接流（v₁）在中心快度区附近存在明显的摆动（wiggle）；旁观核子遮蔽效应（再散射和再吸收）对π的流影响很大；π的横向流在2fm左右反向（相对于核子的流）。完全对心碰撞时，由于纵向对称，核子和π的直接流（v₁）均为0，而椭圆流（v₂），在对心头对头碰撞中，由于横平面对称，椭圆流为0，在对心体对体碰撞中，横平面不对称，椭圆流（v₂）不为0，为最大值约12％；π的流明显小于核子的流；中心快度区由于相互作用最为激烈，椭圆流最强；低橫动量核子和π行为符合流体力学。利用对心体对体和头对头UU碰撞中，明显不同的核子椭圆流，可以有效判选极感兴趣的中心头对头UU碰撞事件。更多还原

【Abstract】 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 baryons△and 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.更多还原