RHIC能量扫描项目中心边缘碰撞可识别强子产额比的实验研究

It has been well accepted that a new kind of quark matter is created in the most central heavy ion collisions at center-of-mass energy-per-nucleon-pair of 200 GeV at RHIC. This matter is often called by theorists Quark-Gluon-Plasma (QGP), which is dominated by quark and gluon degree of freedom. The QGP is assumed to also exist in the early Universe after a few tens microseconds of so called Big Bang. According to the Lattice QCD calculations, the transition from hadronic phase to the QGP phase at high temperature and small baryon chemical potential region is a smooth cross-over, while being a first order phase transition at large baryon chmical potential region. The end point of the first order phase boundary to the cross-over is normally called QCD Critical Point (CP). Lots of work have been done from both sides of the theorists and experimentalists to search for this CP, but there is no universal agreement even on its existence yet. It is therefore important to measure related physics observables at a very broad range of temperature by varying the collision energy levels. From the year 2010, the Relativistic Heavy Ion Collider (RHIC) in BNL, USA initiated a Beam Energy Scan (BES) program and Au+Au collisions have been run at multiple energy levels from 39 down to 7.7 GeV. This program allows us to probe a broad region of the phase structure of nuclear matter and specifically the properties of QGP. While studying the single charged particle spectra at 200 GeV at RHIC, the RAA defined as the ratio of spectra normalized by the number of nucleon collisions, is observed to be consistent with unity at a broad pT regime by comparing the most peripheral Au+Au collisions to those from proton-proton data, providing a reliable basis of vacuum or "non-QGP" reference. The R_{AA} of the most central 200 GeV Au+Au data at RHIC, however, show a strong suppression pattern above pT~2GeV/c. This indicates the effect of QGP where a parton may lose its energy by interacting with the medium. Thus it will be very important to continue measuring the difference between the most central and most peripheral Au+Au data, namely R_{CP}, at all the collected BES energy levels. Both theory models and previous experiments such as SPS have proved that at very low energy this R_{CP} will show an enhancement from such as Cronin effects and radial flow. It is of great interest then to find where this enhancement transits into the suppression at 200 GeV, and to probe the details of R_{CP} around such transition energy. In our plan, we will measure this RCP at all available energy of BES program, as a function of centrality and pT, and use both inclusive and identified charged particles. Comparisons to theory models such as AMPT and UrQMD will also be included. Such measurements will be great help to search for the QCD critical point and study of the bulk properties of nuclear matters.

强相互作用物质的QCD相图是高能核物理领域的一个热门方向，而美国布鲁克海文实验室（BNL）的基于相对论重离子对撞机（RHIC）的束流能量扫描（BES）项目则为在大尺度的范围内研究QCD相图提供了有效途径。在高能量碰撞一端，RHIC 200 GeV 金金碰撞已经给出了粒子产额的强烈的淬火表现，并在理论上解释为夸克胶子等离子体（QGP）和部分子的相互作用。而在低能量碰撞一端，SPS等试验的几个GeV碰撞能量的粒子产额表达出明显的增益，体现了Cronin效应和径向流理论的影响。因此粒子产额随碰撞能量的变化趋势将有力地揭示碰撞产生的核物质的具体性质在QCD相图中的演化趋势。本项目通过对BES项目数据的充分应用，得到粒子产额随碰撞能量从200直至7.7 GeV的详细变化趋势，并辅以对中心度和粒子组分的依赖性的研究，从而探索重离子碰撞末态从夸克相到强子相的变化，并期望对寻找QCD相变临界点作出贡献。

本项目的主要研究方向是通过分析RHIC-STAR束流能量扫描项目的数据，包括金－金碰撞7.7，11.5，14.5，19.6，27，39 GeV 能量，使用可识别强子主要是pion/Kaon/(anti-)proton，研究这些能量的金－金碰撞所产生的夸克物质的整体性质，并与已知的通常被认为产生了夸克胶子等离子体（Quark-Gluon-Plasma，即 QGP）的更高能量碰撞例如RHIC 200 GeV 金－金碰撞数据等进行对比，以获取高温高密度物质相图的完整物理图像。主要物理观测量包括在中心快度区的粒子产额dN/dy，微分横动量谱和谱的一阶二阶矩，可识别粒子和全体粒子的关联方程，以及这些观测量针对各种可识别粒子的比值。通过深入分析这些可观测量随着碰撞能量和中心度的演化，我们获取了介质系统化学和动力学冻析（freeze-out）的相关信息。.目前得到的初步结论包括：..低碰撞能量下增强的质子/反质子比值体现出更强烈的重子停止效应；..低碰撞能量下更高的 pi 介子/反pi 介子比值体现出核自旋和共振态衰变的贡献；..在 7.7 GeV 的 K 介子/pi 介子 和 质子 / pi 介子的比值都达到最高峰，这体现了重子密度在该碰撞能量达到了最大值；..在束流能量扫描项目所使用的碰撞能量范围，平均横动量<pT> （或者<mT>-m）对碰撞能量的依赖较小，而在更小或更大的碰撞能量都有较强的依赖性，这体现了该能量范围的重离子碰撞产生的是一种基于QGP（夸克胶子等离子体）和强子物质的混合相。..我们期望这些实验数据分析成果能够帮助对不同理论模型进行判断筛选，并对不远将来的RHIC束流能量扫描项目第二阶段（2018-2020）提供指导意义。..同时，项目组与理论物理同行合作，产生了大量的唯像数据。这些数据在产生过程中进行了各类模型参数的调整，包括强子生成方式（部分子碎裂 vs. 夸克聚合），部分子作用截面，多重散射过程是否开启。通过对比实验数据和各种配置下的唯像数据，我们能够验证不同的理论模型是否在真实物理过程中存在并发挥主导作用。