RHIC能区重离子碰撞中守恒荷涨落以及QCD相变临界点的实验研究

It is hot and frontier research topic in the high energy theory and experiment to explore the phase structure of strongly interacting nuclear matter as well as searching for the QCD critical point. The phase structure of strongly interacting hot dense nuclear matter can be displayed in the two dimensional Quantum Chromodynamics (QCD) phase diagram (Temperature T Vs. Baryon chemical potential μB). First principle Lattice QCD calculations show that it is a smooth crossover from hadronic to Quark Gluon Plasma (QGP) phase at vanishing baryon chemical potential and finite temperature. At large μB region, theoretical calculation demonstrates that the phase transition is of the first order. Thus, there should have a so called QCD Critical Point (CP) as the end point of the first order phase transition boundary towards the crossover region. Due to the sign problem of the Lattice QCD at finite μB, there is large uncertainty in determining the location of the CP or even its existence. Experimental confirmation of the existence of the CP will be an excellent verification of QCD theory in the non-perturbative region and a milestone of exploring the QCD phase structure. It is one of the main goal of the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). By tuning the colliding energy of gold nuclei from high to low values, one can vary the temperature and baryon chemical potential of the hot dense nuclear matter created in high energy nuclear collisions. This can provide us a unique experimental tool to probe the QCD phase structure in a broad T and μB region. Fluctuation of conserved quantities, such as net-baryon (B), net-charge (Q) and net-strangeness (S), can serve as powerful observables to probe the QCD phase transition and CP signal in heavy-ion collisions. Because those observables are sensitive to the correlation length of the system and directly connected to the susceptibilities in the theoretical calculations. Theoretically, it is predicted that the net- proton number fluctuation is a reasonable proxy of the fluctuation of net-baryon number in measuring critical fluctuations near the QCD critical point. However, if one considers the effects of the non-critical contributions associated with heavy-ion collisions dynamics, such as hadronic scattering, resonance decay and baryon number conservation, there would be difference between net-proton and net-baryon fluctuations. To understand those non-critical contributions to the observables in terms of searching for the CP, careful studies are necessary.

研究量子色动力学(QCD)描述的强相互作用物质相变结构是当前高能物理理论和实验的热点和前沿。美国相对论重离子对撞机(RHIC)上能量扫描计划的一个主要目标就是通过改变金核金核碰撞的质心能量来从实验上研究热密核物质相变以及寻找理论预言存在于QCD相图高 重子密度区的QCD相变临界点。该临界点的实验确认将是对QCD理论在非微扰区域的一个绝佳验证，是研究QCD相结构的里程碑，具有重要科学意义。理论预言高能重离子碰撞中逐事件守恒荷，如净重子，净电荷与净奇异数，的高阶涨落对系统关联长度非常敏感，并且与守恒荷对应的热力学感应率直接关联。因此为在重离子碰撞中观测热密核物质相变以及QCD临界点提供了理想的实验探针。本项目旨在利用从RHIC能量扫描中STAR探测器采集到的实验数据，对净质子，净K介子以及净电荷数的逐事件涨落进行系统的分析和测量，为研究QCD相变以及寻找QCD临界点的提供实验依据和参考。

研究强相互作用物质相结构、寻找QCD临界点是当今国际核物理实验以及理论研究的热点和前沿课题。QCD临界点的实验确认是探索强相互作用物质相结构的里程碑，将为研究高重子密度区QCD相图以及检验QCD理论提供重要实验依据，具有重要科学意义。因此它被写入中国学科发展战略核物理部分首个待解决的科学问题。为了在这一具有潜在重大发现的研究方向上占据领先地位、取得突破，各国陆续建造大型粒子探测器(包括：美国RHIC-STAR实验， 德国CBM实验、俄罗斯 NICA实验、日本J-PARC实验以及中国兰州CSR外靶CEE实验)，开展重离子碰撞实验，其主要目标就是研究强相互作用物质相图结构、寻找QCD临界点。本项目主要是基于RHIC-STAR实验采集的第一阶段能量扫描的实验数据，对净质子，净K介子以及净电荷数的逐事件涨落和轻核的产生进行系统的分析和测量。在本项目的支持下，项目主持人带领研究生以及博士后对STAR 采集的多个金核金核碰撞能量点下的实验数据进行了系统分析，主要在三个方面取得了进展以及代表性成果： 1. 首次观察到，净质子数分布的四阶矩对碰撞能量出现非单调依赖。暗示碰撞系统可能进入了临界区。该实验结果需要进一步高统计测量结果确认。2. 对RHIC第一阶段能量扫描实验数据中轻核的产生(氘、氚)进行了系统的测量。首次分析得到了 QCD临界点敏感观测量--轻核产额比对碰撞能量的非单调依赖，并在20-30 GeV能量附近观察到一个峰值。3. 在RHIC金金碰撞质心能量200 GeV高统计实验数据中，对净质子数分布的六阶矩进行了首次测量。在0-40%中心度碰撞的实验结果中观察到负值，该实验结果为研究低重子化学势下QCD手征相变提供了重要实验参考。这三个代表成果均在本领域最高级别会议上代表STAR合作组给了口头报告。第一个成果文章已经投稿。另外两个研究成果，文章已撰写完成，正在合作组内部审核中。围绕这些实验测量，相关的模型以及实验方法的研究也取得了一些进展，并发表多篇期刊文章。