RHIC能量扫描中逐事件净质子数分布的高阶矩实验研究

The phase structure of nuclear matter, described by strong interaction, can be demonstrated by QCD phase diagram, which is two dimensional diagram with parameters temperature and baryon chemical potential. A new form of matter-Quark Gluon Plasma (QGP) dominated by quark and gluon degree of freedom is believed to exist in the early Universe after few tens microseconds of Big Bang, when the energy density and temperature is extremely high. Finite temperature Lattice QCD calculations predict a smooth cross-over transition from hadronic phase to the QGP phase at high temperature and small baryon chemical potential region, and a first order phase transition at large baryon chemical potential region. The end point of the first order phase boundary toward the cross-over region is the so called QCD Critical Point (CP). Although many efforts have been made by theorist and experimentalist to locate the CP, its location or even existence is still not confirmed yet. In the year 2010, The Relativistic Heavy Ion Collider (RHIC) in BNL, USA has started its Beam Energy Scan (BES) program and tuned the Au+Au collision energy from 39 down to 7.7 GeV. This allows us to access and probe broad region of the QCD phase diagram and provides us an ideal experimental tool to explore the phase structure of nuclear matter and study the properties of QGP. Recently, it was found that the higher moments (Variance,Skewness,Kurtosis) of conserved quantities, such as net-baryon,net-charge, and net-strangeness, distributions can be directly connected to the corresponding thermodynamic susceptibilities. On the other hand, theoretical calculations demonstrate that the experimental measurable net-proton (proton number minus anti-proton number) number fluctuations can effectively reflect the fluctuations of the net-baryon and net-charge number in heavy ion collision experiments. Thus, it is of great interest to measure the higher moments of event-by-event net-proton multiplicity distributions in the heavy ion collision experiment. It allows us to probe the bulk properties of the hot dense nuclear matter and test the QCD theory at non-perturbative domain, which is rarely tested by experiments. Meanwhile, model calculations demonstrate that the higher moments of net-proton distributions are also proportional to the higher power of the correlation length. It motivates us to search for the QCD critical point with the higher moments of the net-proton distributions, experimentally, as a direct application of the higher moments observable.In our plan, we will present comprehensive and systematical measurements with RHIC-STAR detector for the higher moments of net-proton distributions in the BES program. The experimental results will be applied to search for the QCD critical point and study the bulk properties of nuclear matters.

美国布鲁克海汶国家实验室(BNL)中相对论重离子对撞机(RHIC)上进行的能量扫描计划的主要目的之一就是通过改变金核金核碰撞质心能量来探索更大范围的QCD物质相图区域，这为实验研究强相互作用物质的相变结构提供了一种有效的途径。理论研究表明，高能重离子碰撞实验中可观测的逐事件净质子数（质子数减去反质子数）分布的高阶矩对系统的关联长度非常灵敏并且与系统重子数的热力学感应率有直接的关联。这为研究QCD物质性质以及相变结构提供了很好的实验探针。本项目旨在利用从RHIC上的STAR探测器采集到的能量扫描实验数据，对逐事件净质子数分布的高阶矩进行系统的测量和分析，并将所得实验结果应用到寻找QCD相变临界点的研究。

寻找QCD相图中强子相到夸克胶子等离子体相的相变边界以及QCD临界点成为当今国际高能核物理实验和理论研究的热点和前沿。QCD临界点的实验确认将是探索高温高密核物质相结构的里程碑，具有重大科学意义。我们首次系统地在美国RHIC-STAR实验中开展了逐事件净质子数的高阶涨落研究，并在第一阶段RHIC重离子碰撞能量扫描中寻找QCD临界点的特征信号，即观测量对碰撞能量的非单调依赖。 结果表明在最中心碰撞中，观测量相对于统计涨落基线显示出对碰撞能量的非单调依赖，在碰撞能量为19.6GeV附近，对该基线有较大负偏离，形成了一个极小点。暗示了附近能量点可能已经进入到了“临界区”。由于此研究的重要性，RHIC准备在2019-2020年进行第二阶段高统计量的能量扫描，确认相变临界点是否存在。发表论文8篇(其中PRL一篇, PRC一篇)、并在“夸克物质”、“临界点和QCD相变”等10个国际会议作大会邀请报告。