【作者】 施梳苏；

【导师】 刘峰； 许怒；

【作者基本信息】 华中师范大学 ， 粒子物理与原子核物理， 2010， 博士

【摘要】 在相对论重离子碰撞的早期,产生出了一种高温高密的强相互作用的新物质形态。随后系统的演化则由新物质的性质决定。在本篇论文中,我们通过测量重离子碰撞中产生的粒子相对于反应平面(由碰撞参量和束流所在的方向决定)的方位角各向异性来研究系统演化的动力学。在非对心碰撞中(碰撞参数不为零),参加碰撞的区域成一个椭球状,因而在空间坐标中是各向异性的。这种初始的空间坐标中的各向异性会通过相互作用而转化为末态动量空间的各向异性。本文中所讨论的椭球流,v2,是末态动量空间粒子方位角分布傅里叶展开式的第二项。由于初始的空间各向异性所产生的压力梯度随时间演化迅速消失(自猝灭效应),椭球流直接于碰撞早期的动力学相联系。椭球流的大小强烈依赖于碰撞过程中相互作用的强弱,因此它可以提供早期重离子碰撞中压力梯度,有效自由度,热化以及碰撞早期新物质形态状态方程的信息。进一步,学习椭圆流在不同对撞能量下对中心度和系统大小的依赖性是研究高能重离子碰撞产生的新物质形态性质的有效途径。在这篇论文中,我们系统分析了由STAR探测器在2005年RHIC运行中所采集的质心系能量为62.4 GeV和200 GeV的铜铜碰撞数据和在2007年所采集的9.2 GeV和200GeV金金碰撞事件。我们测量了金金200 GeV碰撞事件中奇异粒子的椭球流。借助于2007年采集的金金200 GeV大统计量数据,我们可以精确的测量到多重奇异粒子Φ介子和Ω重子的椭球流。由于多重奇异粒子产生于高能重离子碰撞的早期,而且它们具有较小的强子强子相互作用截面,因此多重奇异粒子被认为是能够反映碰撞早期物理的探针。Φ介子和Ω重子显著的椭球流为部分子层次的集体运动在RHIC的形成提供了强有力的证据。其中在中横动量区(2<r< 5 GeV/c)观察到的组分夸克标度性说明夸克层次的自由度已经达到,即在强子化过程之前存在一个解禁闭的状态。我们利用STAR采集的不同系统和不同碰撞能量的数据,系统的研究了组分夸克的标度性,发现在62.4 GeV和200 GeV的金金与铜铜碰撞事件中,都能清楚的观察到这一标度性。实验结果表明,在质心系能量为62.4 GeV和200 GeV的重离子碰撞中所产生的物质达到了解禁闭状态。我们测量了铜铜200 GeV碰撞事件中带电粒子和奇异粒子的椭球流。由于铜铜碰撞系统相比较于金金系统小的多,因此具有较小的末态多重数。在这种情况下,如果我们用中间快度区(|η|<1.0)所确定的反应平面测量椭球流则会包含较大的非流效应。我们首次采用了用前端快度区的粒子确定反应平面测量椭球流的方法,使得非流效应在铜铜碰撞系统中所带来的系统误差大大降低。利用相同能量下P+P碰撞数据,我们分析了椭球流测量中的剩余非流效应。通过对带电强子以及奇异粒子(KS0,Λ+(?)and三一+(?)+)在不同对心度的椭球流测量并于金金碰撞中的数据相比较,我们研究了椭球流对碰撞系统和对心度的依赖性。我们发现奇异粒子的椭球流相对于横动量的依赖性与金金碰撞中所表现出来的标度性十分相似：(i)在低横动量区,pT<2 GeV／c,椭球流具有横能量,mT-m,的标度性；(ii)在中横动量区,2<pT<4 GeV／c,椭球流具有组分夸克的标度性。碰撞系统的初始条件可以由碰撞区域的偏心率(ε)来表示,我们发现不同对心度的椭球流除以其对应的初始几何条件之后,在越接近对心的碰撞中可观察到较大的椭球流,这表明集体运动的强弱依赖于对心度。通过对不同对心度及不同碰撞系统的比较,我们发现椭球流的强弱依赖于一个共同的量,参加反应的核子数(Npart).在理想流体力学极限下,由于系统充分热化,椭球流由初始几何形状完全决定。因此这一实验结果表明理想流体极限在铜铜碰撞中没有达到,可能是因为完全热化这一假设并不成立。由于理想流体力学的基本假设是系统完全热化,因此比较椭球流的实验结果与理想流体的计算结果有助于我们研究RHIC能量下重离子碰撞早期所形成的物质是否完全热化。我们的研究表明,理想流体力学的椭球流对横动量的依赖性的计算结果在不同对心度中与实验结果不吻合。迄今为止,用于理想流体计算的模型仍有一些因素没有考虑,诸如初始几何形状的逐事件涨落(这一效应尤其对中心碰撞的结果影响较大),有限的粘滞系数等。因此,进一步的理论研究需要考量这些因素是否导致了我们所观察到的实验数据与理论计算的不一致。利用输运模型,我们分析了v2／ε对末态粒子密度的1/S dN/dy依赖性。我们的结果表明,在接近中心的金金200 GeV碰撞中,系统只达到了0.46-0.07+0.24或者0.75-0.10+0.14理想流体力学极限,不同的计算结果分别基于确定碰撞初始条件的模型Glauber与CGC。进一步,通过输运模型所提取的Knudsen参数使得我们可以估算有效的部分子散射截面以及剪切粘滞系数相对于熵密度的比值。在2008年的测试运行中,STAR探测器成功的采集了三千个金金9.2 GeV碰撞事件。利用这些事件,我们测量了带电强子,π介子和p重子的椭球流。我们的结果与相同碰撞能量相似碰撞系统NA49实验组的发表结果吻合。这表明,STAR探测器在较低能量下运行良好,具有进行束流能量扫描的能力。在即将进行的RHIC能量扫描的运行计划中,通过对椭球流组分夸克标度性的碰撞能量依赖性的研究,我们可以确定形成解禁闭物质系统的碰撞能量,这对寻找QCD预言的临界点和相边界有重要的意义。更多还原

【Abstract】 At the early stages of relativistic heavy ion collisions, a hot and dense, strongly interact-ing medium is created. The subsequent system evolution is determined by the nature of the medium. Experimentally, the dynamics of the system evolution has been studied by measuring the azimuthal anisotropy of the particle production relative to the reaction plane. The centrality of the collision, defined by the transverse distance between the cen-ters of the colliding nuclei called the impact parameter, results in an "almond-shaped" overlap region that is spatially azimuthal anisotropic. It is generally assumed that the initial spatial anisotropy in the system is converted into momentum-space anisotropy through re-scatterings. The elliptic flow, v2, is the second harmonic coefficient of a Fourier expansion of the final momentum-space azimuthal anisotropy. Due to the self-quenching effect, it provides information about the dynamics at the early stage of the collisions. Elliptic flow can provide information about the pressure gradients, the effec-tive degrees of freedom, the degree of thermalization, and equation of state of the matter created at the early stage. Thus, the centrality and system-size dependence of elliptic flow at different beam energies can be used to study the properties of the matter created in heavy ion collisions.In this thesis, we analyze the data collected with the STAR detector from (?)= 62.4 and 200 GeV Cu+Cu collisions during the fifth RHIC run in 2005 and (?)= 9.2 and 200 GeV Au+Au collisions during the seventh run in 2007. We present results on elliptic flow v2 of identified particles in Au+Au collisions at (?)= 200 GeV. With the large statistics of the RHIC seventh run in 2007, we measured multi-strange hadrons, (?) and Q v2 in high precision. We find they flow almost as strong as pion and proton. As multi-strange hadrons are created at the early stage of the collisions, and they are less sensitive to the late hadronic process with their smaller hadronic cross section, thus, the significant v2 of multi-strange hadrons indicates the partonic collectivity has been built up in the heavy ion collisions at RHIC. The Number of Quark (NQ) scaling reflects constituent quark is the most effect degree of freedom in determining hadron flow at intermediate pT.This suggests that the system has been in the deconfined state prior to hadronization. We systematically discuss the NQ scaling at RHIC and find it holds in the intermediate pT region,2<pT< 5 GeV/c, for all systems (Au+Au and Cu+Cu) and beam energies (62.4 GeV and 200 GeV). It suggests the deconfinement has been reached at RHIC.We present the results of an elliptic flow analysis of Cu+Cu collisions at (?)= 62.4 and 200 GeV. Elliptic flow as a function of transverse momentum, V2(pT), is reported for different collision centralities for charged hadrons h±, and strangeness containing hadrons KS0, A+(?) andΞ-+(?)+ in the midrapidity region |η|< 1.0. Significant reduction in systematic uncertainty of the measurement due to non-flow effects has been achieved by correlating particles at midrapidity,|η|< 1.0, with those at forward rapidity,2.5< |η|< 4.0. We also present azimuthal correlations in p+p collisions at (?)= 200 GeV to help estimating non-flow effects. To study the system-size dependence of elliptic flow, we present a detailed comparison with the results from Au+Au collisions at (?)= 200 GeV. We observe that v2(pT) of strange hadrons has similar scaling properties as were first observed in Au+Au collisions, i.e.:(i) at low transverse momenta, pT< 2 GeV/c, v2 scales with transverse kinetic energy, mT-m, and (ii) at intermediate pT,2< pT< 4 GeV/c, it scales with the number of constituent quarks, nq. Eccentricity scaled v2 values, v2/ε, are larger in more central collisions, suggesting stronger collective flow develops in more central collisions. The comparison with Au+Au collisions which go further in density shows v2/εdepend on the system size, number of participants Npart. This indicates that the ideal hydrodynamic limit is not reached in Cu+Cu collisions, presumably because the assumption of thermalization is not attained.The comparison of the data to the ideal hydrodynamic calculations may shed light on the thermalization issue at RHIC. We find that ideal hydrodynamic calculations fail to reproduce the centrality dependence of V2{pT) in both Au+Au and Cu+Cu collisions at (?)= 200 GeV collisions. To date, there are serval effects not included in the model, such as geometrical fluctuations in the initial conditions (particularly important in central collisions), finite viscosity effects. It remains to be seen if these effects can account for the difference between the models and data.With a transport model, we study the 1/S dN/dy dependence of v2/ε. The extracted Knudsen numbers show finite values, even for central collisions. It indicates that the system has reached 0.46-0.07+0.24 and 0.75-0.10+0.14 of ideal hydrodynamic limits, using Glauber and Color Glass Condensate (CGC) initial condition, respectively. The lack of perfect equilibration allows for estimates of the effective parton cross section in the quark-gluon plasma and of the shear viscosity to entropy density.With 3 k events collected using STAR detector from a test run of the collider in the year 2008, we present the results of an elliptic flow analysis of Au+Au collisions at (?)= 9.2 GeV. Our results are consistent with the corresponding previous results from NA49 at similar (?).It demonstrates the capabilities of the STAR detector to pursue the proposed beam energy scan. The beam energy dependence of NQ scaling in v2 should be a powerful tool for searching for the possible QCD phase boundary.更多还原