高能重离子碰撞各向异性流纵向涨落的能量和系统尺寸依赖性研究

A strongly interacting quark-gluon plasma (QGP) composed of deconfined partons is believed to be formed experimentally in relativistic heavy-ion collisions. In a non-head-on collisions, the transverse overlap region of colliding nuclei is anisotropic. Due to interactions among constituents, the created matter undergoes a rapid anisotropic expansion, resulting in an observable momentum-space azimuthal anisotropy. Elliptic flow (v2) is measured by the second coefficient of the Fourier expansion of the final-state particle azimuthal distribution. Because the anisotropy in configuration space is quickly diminished due to anisotropic expansion, elliptic flow is primarily sensitive to the early stage of sQGP evolution. Shear viscosity is known to damp the development of anisotropic flow. Anisotropic flow data in comparison to hydrodynamical calculations may, therefore, constrain the shear viscosity to entropy density ratio (η/s). Due to fluctuations in the initial transverse energy-density distribution, all the harmonic flows can exist and the harmonic planes of different orders are not necessarily the same. Higher order harmonic flows are thought to be more sensitive to η/s. Therefore, they play an important role in extracting information about the hot and dense medium. Because of longitudinal fluctuations in the initial energy density distribution, the harmonic event planes fluctuate along the longitudinal direction even in a given event, which may lead to longitudinal fluctuations in anisotropic flows through hydrodynamic evolution. Early studies of longitudinal fluctuations in harmonic flows have provided an important tool for understanding the initial conditions and the transport properties of the created QGP in heavy-ion collisions. It has only been started for the studies of the longitudinal dynamics of harmonic flows. The initial energy density distributions are expected to vary significantly with collision energy in heavy-ion collisions. The initial-state fluctuations are possibly larger at lower energy and hence much stronger event plane decorrelations along pseudo-rapidty maybe observed. Recent measurements of two-particle correlations in p+p, Cu+Cu, p+Au and Au+Cu collisions suggest that hydrodynamic behavior may also be present in these small collision systems. The distribution of participant multiplicity exhibits forward-backward symmetry averaging over many events, but this is unnecessarily the case on an event-by-event idea, especially for asymmetric heavy-ion collisions. The asymmetry of participant multiplicity distribution can cause fluctuations in final-state particle multiplicity distribution and maybe the origin of event plane angle fluctuations. Therefore, it would be interesting to study the longitudinal collective dynamics in these collisions, where the harmonic event plane fluctuations may be much larger than for A+A collisions. In our plan, we will present comprehensive and systematical measurements with collision energy and system size dependences of longitudinal fluctuations in anisotropic flows within transport models. Event plane decorrelations over pseudo-rapidity varying with collision energy, centrality and system size will be measured, and the properties of longitudinal fluctuations in anisotropic flows will be systematically studied. Our study will have the potential to greatly improve our understanding of the initial conditions and dynamical evolution of the QGP in heavy-ion collisions.

寻找并研究新物质形态夸克胶子等离子体(QGP)是相对论重离子碰撞主要目标之一。各向异性流形成于重离子碰撞早期，敏感于组分之间相互作用机制与强弱，能提供媒质热化、压力梯度以及状态方程等信息。核核碰撞初始能量密度分布纵向涨落将产生事件平面分布去相关性，碰撞系统以流体力学形式演化，该效应将引起各向异性流分布纵向涨落，各向异性流纵向涨落的研究为理解热密媒质初始条件、输运性质和动力学演化过程具有重要作用。本项目旨在利用相对论重离子碰撞输运模型数据，测量事件平面纵向关联对核核碰撞能量、中心度和系统尺寸依赖关系，研究事件平面分布纵向去相关性对各向异性流测量影响，进而对重离子碰撞中逐事件各向异性流纵向涨落性质进行系统分析。此工作将为RHIC和LHC实验数据的物理分析提供输运理论支持。

相对论重离子对撞机（RHIC）的主要目标为寻找并研究新物质形态夸克胶子等离子体(QGP)。核核碰撞初始能量密度涨落将引起各向异性流纵向涨落，各向异性流纵向涨落的研究为理解热密媒质初始条件、输运性质和动力学演化过程发挥了重要作用。基于AMPT和UrQMD模型的Au+Au碰撞能量扫描数据，测量了不同碰撞中心度的事件平面分布以及事件平面纵向关联强度相对碰撞中心度、碰撞能量的依赖性。研究表明第二阶事件平面纵向关联强度随碰撞中心度呈现非单调变化，而第三阶事件平面纵向关联强度不敏感于碰撞中心度。通过分辨率修正后的第二阶和第三阶事件平面纵向关联强度小于单位一，这意味着Au+Au碰撞能量扫描数据中存在事件平面纵向关联去相关性。事件平面纵向关联强度依赖于碰撞能量，随着碰撞能量降低，事件平面纵向关联强度减小，这说明较低能量的核核碰撞将产生较强的事件平面纵向关联去相关性。测量了各向异性流因子化比值随赝快度分布，以及比较了Cu+Au和Au+Au碰撞中第二阶和第三阶事件平面纵向关联强度随赝快度间隔分布。各向异性流因子化比值的测量结果也表明事件平面存在纵向关联去相关性，研究结果定性上与流体力学计算和CMS实验数据一致。与Au+Au碰撞的结果相比，Cu+Au碰撞的事件平面纵向关联去相关性呈现较快的下降趋势，这可能与Cu+Au碰撞中前向和后向粒子分布非对称性有关。