【作者】 吴凤娟；

【导师】 霍雷；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2009， 博士

【摘要】 重离子碰撞物理研究的主要目的之一是探索高温高密极端条件下核物质的性质,获取核物质的状态方程。高能重离子碰撞中核物质集体流是大量粒子的集体运动,对碰撞早期阶段形成的压缩核物质的性质敏感,因此能为获取核物质状态方程提供非常重要的信息。在GeV能区的重离子碰撞中,由于单个事件的粒子数有限,为了减小统计涨落的影响,目前的集体流研究方法中,一般是将大量事件叠加到一起进行分析。但是,在这样的叠加平均过程中,许多重要的信息被湮没了。本论文利用相对论量子分子动力学模型(RQMD)模拟AGS能区Au+Au碰撞,研究单个事件中侧向流和椭圆流之间多粒子方位角集体关联,探索椭圆流的形成机制,研究椭圆流的时间演化及反应过程中的空间-动量关联。介绍了获取集体流常用的估计反应平面法,利用该方法分析了RQMD模型模拟产生的AGS能区Au+Au碰撞中侧向流和椭圆流随碰撞参数、入射能量及末态粒子横动量的变化,并且讨论了集体流的形成机制。结果表明,利用侧向流确定的估计反应平面方法能一定程度地再现模型中真实流值。在不需要确定估计反应平面的基础上,利用单个事件中侧向流和椭圆流之间多粒子方位角集体关联来描述核物质运动的集体性,建立了多粒子方位角关联函数。引入多粒子非对称性参量表征单个事件中侧向流和椭圆流之间多粒子集体关联强度,该变量还可以反映出侧向流和椭圆流的相对取向。利用多粒子方位角关联函数分析了AGS能区Au+Au碰撞中核子与核子之间以及核子与π介子之间多粒子方位角集体关联随碰撞参数和入射能量的变化。研究表明,多粒子非对称性参量能很好地描述重离子碰撞中的核物质运动的集体性。利用多粒子方位角集体关联能够对碰撞事件进行分组研究,克服了GeV能区单个事件中粒子数有限的限制。对不同事件组的分析结果表明,AGS能区Au+Au碰撞中末态动量空间非对称性与初始坐标空间非对称性有关,系统的动力学演化过程中存在明显的空间-动量关联。在AGS能区,椭圆流的产生受平均场,两体碰撞和旁观者区域的屏蔽等各种因素的影响。利用RQMD模型分别计算了是否有旁观者区域时AGS能区Au+Au碰撞中核子和π介子的椭圆流随碰撞参数、入射能量以及末态粒子横动量的变化。研究结果表明,在AGS能区Au+Au碰撞中,旁观者区域的屏蔽作用会对核子和π介子的椭圆流产生影响,这种影响随入射能量的增加而逐渐减小,而随着碰撞参数的增加而增加。旁观者区域屏蔽作用对椭圆流的影响在大横动量区域的表现更加明显。更多还原

【Abstract】 One of the main goals of heavy ion collisions is to study nuclear matter under ex-treme conditions of high temperatures and high densities, i.e. to learn more about thenuclear equation of state. Collective ?ow of the nuclear matter in high energy heavy ioncollisions is a multi-particle collective motion and it is very sensitive to the maximumcompression reached in the early phase of a heavy ion reaction and hence can providean important probe for the nuclear equation of state. At GeV energies, due to the finitenumber of particles in an event, one can put many events together for the ?ow analyses toreduce the effects of statistical ?uctuations. However, much important information maybe lost in the averaging process. In this dissertation, we simulate the reactions of Au+Aucollisions using RQMD model to investigate multi-particle azimuthal collective correla-tions between the directed and elliptic ?ow in an event, explore formation mechanism ofthe elliptic ?ow, and study the time evolution of elliptic ?ow and the space-momentumcorrelations during the process of the collisions.The commonly used method of determining the collective ?ow of the estimated reac-tion plane is introduced. The directed and elliptic ?ow based on the method as a functionof the impact parameter, the incident energy and the transverse momentum of the finalstate particles are analyzed in Au+Au collisions at AGS energies simulated with RQMDmodel. The formation mechanism of the collective ?ow is also discussed. The calculatedresults by the estimated reaction plane can reproduce to some extent the real values in themodel. Without determining the estimated reaction plane, the multi-particle azimuthalcollective correlation between the directed and elliptic ?ow in an event can describe thecollectivity of nuclear matter in heavy ion collisions, and multi-particle azimuthal corre-lation function is established.A variable of multi-particle anisotropy to characterize the strength of multi-particlecollective correlation is introduced, and it can also re?ect the relative orientation betweenthe directed and elliptic flow. We use multi-particle azimuthal correlation function to an-alyze the multi-particle collective correlations between the nucleons and nucleons, andbetween the nucleons and pions as a function of the impact parameter, the incident en-ergy in Au+Au collisions at AGS energies. Our results show that the variable of multi- particle anisotropy can well describe the collectivity of the nuclear matter. We divide theevents into different groups according to the multi-particle collective correlations to over-come the finite numbers of particles in an event at GeV energies. The results in differentgroups show that the final anisotropy in the momentum space is correlated with the initialanisotropy in coordinate space in Au+Au collisions at AGS energies, and there are clearspace-momentum correlations during the whole dynamical evolution of the system.At AGS energies, elliptic flow is affected by the mean field, two body collisionsand the shadowing effects of the spectator regions, etc. We calculate the elliptic flow ofnucleons and pions as a function of the impact parameter, the incident energy and thetransverse momentum of the final state particles in Au+Au collisions with and withoutthe spectator regions at AGS energies. The results show that the shadowing effects ofthe spectator regions on the elliptic ?ow of nucleons and pions are obvious in Au+Aucollisions at AGS energies. The shadowing effects decrease with the increasing incidentenergy and increase with the increasing impact parameter. The shadowing effects of thespectator regions on the elliptic ?ow in the large transverse momentum region are moreobvious.更多还原