RHIC固定靶能区净质子数分布的高阶矩实验研究

量子色动力学(QCD)相图结构和相变临界点是高能物理理论和实验的研究热点。相对论重离子碰撞是探索QCD相图结构、寻找QCD相变临界点的有力工具。美国布鲁克海文国家实验室的相对论重离子对撞机(RHIC)是目前世界上进行高能重离子碰撞的大型实验装置之一，其中的STAR实验致力于高温高密条件下夸克胶子等离子体(QGP)性质以及QCD相结构的实验研究。理论预言，逐事件守恒量(如净重子数、净奇异数以及净电荷数)分布的高阶累积量是重离子碰撞中寻找临界点的敏感观测量。本项目将分析RHIC-STAR采集固定靶实验数据，测量守恒荷高阶矩，研究寻找QCD临界点和相边界。

Experimental evidences at RHIC and the LHC have demonstrated the formation of Quark-Gluon Plasma (QGP) in ultra-relativistic heavy-ion collisions at small baryon chemical potential (μB ≈ 0 MeV) where the phase transition from the hadronic matter to QGP is suggested to be a crossover from state-of-the-art Lattice QCD calculations. It has been conjectured that there is a first-order phase transition and a critical point at finite μB region in the QCD phase diagram. Higher-order cumulants of conserved quantities (Baryon number, Electric charge, and Strangeness) have been suggested as sensitive observables to search for the critical point and phase boundary in the QCD phase diagram. 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 a 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 this program, we will perform comprehensive and systematical measurements on higher-order cumulants of net-proton distributions using RHIC-STAR fixed-target experimental data. The measurements will be crucial to the search for the QCD critical point.