相对论重离子碰撞中背向强子关联研究

A strongly interacting Quark Gluon Plasma (QGP) is created in relativistic heavy ion collision at the Relativistic Heavy Ion Collider (RHIC). Large transverse momentum (pT) partons in the high-density system result from the initial hard scattering of nucleon constituents. After a hard scattering, the parton fragments to create a high-energy cluster (jet) of particles. The energetic parton is predicted to experience substantial energy loss when traversing the dense colored medium and therefore can be used a probe to detect the medium property. Since the medium interaction probability increases with path length through the medium, high-pT particle production is strongly biased towards surface of the collision zone. The study of single particle production at high pT gives limited information about the interior of the QCD medium and about jet-medium interactions. However, the away-side jet partner opposite to a high-pT particle traverses the entire volume suffering maximal interactions with the medium. Therefore, di-hadron and multi-hadron correlations with high pT trigger particles can be used as valuable.tools to study the energy loss mechanisms due to interactions of energetic partons with the QGP medium...A novel phenomenon has been observed in di-hadron correlations: the double-peak correlation structure in azimuthal angle on the away side (di-hadron azimuthal angle difference larger than π/2). The leading explanation of the double-peak structure is Mach-cone shock wave excitations emitting particles in a preferred direction dependent of the speed of sound of the medium. However, only the elliptic flow v.2 background is subtracted in these correlation measurements. The away-side double-peaked correlations maybe mainly caused by higher order hydrodynamic flow harmonics, particularly v3 that have been overlooked until recently. The subtraction of the elliptic flow v2 background involves determination of the elliptic flow parameters v2 and the amplitude of the flow background (normalization). The v2 parameters are.from separate flow measurements. The elliptic flow background has been normalized to the data around |\Delta\phi| = 1 assuming that the background-subtracted signal is zero at minimum. This is referred to as the ZYAM (zero yield at minimum) method. Those measurements are usually coming with large uncertainties...We devise a novel method to subtract the flow background using data. No assumptions on the flow shape and amplitude are made and we are going to subtract all orders of flow background. The away-side jet correlation width is studied in Au+Au collisions at √sNN =200 GeV as a function of centrality and associated particle pT. The width is found to increase with centrality at modest to high associated particle pT. The increase can arise from jet-medium modifications, event averaging of away-side jets deflected by medium flow, and/or simply nuclear kT broadening. To further discriminate various physics mechanisms, a three-particle correlation analysis is conducted with robust flow background subtraction also using data. Based on the three-particle correlation analysis we discuss possible physics mechanisms of away-side broadening of jet-like correlations.

相对论重离子对撞机（RHIC）上进行的重离子碰撞实验产生了一种被称为夸克胶子等离子体（QGP）的物质形态。QGP的寻找和探索对增强强相互作用理论（QCD）的深入理解具有重要意义，也对认识宇宙早期演化的性质有深远影响。碰撞实验中夸克和胶子硬散射产生的高能部分子（Parton）可以被用作探测QGP性质的探针。通过实验末态粒子的关联测量研究高能部分子与QGP的相互作用，是研究QGP性质的重要手段。由于重离子碰撞演化过程的复杂性，强子关联分析中的关键内容和难点在于背景扣除。本项目系统的提出了在双强子关联和三强子关联中用于扣除集体流背景的新方法。这种方法可以可靠的扣除各阶集体流背景。在可靠的背景扣除下研究了高能部分子与夸克胶子等离子的相互作用过程，对于理解部分子在夸克胶子等离子中的能量损失机制和研究夸克胶子等离子体的性质具有重要意义。

相对论重离子对撞机(RHIC)上进行的重离子碰撞实验产生了一种被称为夸克胶子等离子体(QGP)的物质形态。QGP的寻找和探索对增强强相互作用理论(QCD)的深入理解具有重要意义，也对认识宇宙早期演化的性质有深远影响。碰撞实验中夸克和胶子硬散射产生的高能部分子(Parton)可以被用作探测QGP性质的探针。通过实验末态粒子的关联测量研究高能部分 子与QGP的相互作用，是研究QGP性质的重要手段。由于重离子碰撞演化过程的复杂性，强子关联分析中的关键内容和难点在于背景扣除。本项目系统的提出了在双强子关联和三强子关联中用于扣除集体流背景的新方法。这种方法可以可靠的扣除各阶集体流背景。在可靠的背景扣除下研究了高能部分子与夸克胶子等离子的相互作用过程。研究结果显示，在双强子关联测量中背端强子关联函数的宽度随碰撞中心度上升而变宽，进一步研究了在中心碰撞和边缘碰撞中背端强子关联函数宽度的差别随关联粒子横动量的变化，得出了除nuclear kT效应之外高能量部分子与QGP发生相互作用即热核效应的存在的结论。三强子关联测量的结果显示，相对论重离子碰撞中存在很强的nuclear kT效应或者高能量部分子能够被QGP介质弹开的物理机制，而高能量部分子在QGP介质中运动发射马赫波的现象可能受限了目前的统计量并没有被观察到。本研究项目有助于进一步理解高能量部分子在QGP中的能量损失机制。若希望彻底区分目前理论学家提出的几种部分子在QGP中的能量损失机制，还需要进一步的研究。