相对论重离子碰撞中确定QCD相边界的若干问题  

Several problems in determining the QCD phase boundary by relativistic heavy ion collisions

作　　者：吴元芳[1] 李笑冰 陈丽珠 李治明[1] 许明梅[1] 潘雪 张凡 张雁华 钟昱明 WU Yuanfang;LI Xiaobing;CHEN Lizhu;LI Zhiming;XU Mingmei;PAN Xue;ZHANG Fan;ZHANG Yanhua;ZHONG Yuming(Institution of Particle Physics,Central China Normal University,Wuhan 430079,China;School of Physics and Optoelectronic Engineering,Nanjing University of Information Science and Technology,Nanjing 210044,China;School of Electronic Engineering,Chengdu Technological University,Chengdu 611730,China;Wuhan Optical Valley Future School,Wuhan 430078,China;Department of Physics and Electronic Engineering,Yuncheng University,Yuncheng 044000,China)

机构地区：[1]华中师范大学粒子物理研究所,武汉430079 [2]南京信息工程大学物理与光电工程学院,南京210044 [3]成都工业学院电子工程学院,成都611730 [4]武汉光谷未来学校,武汉430078 [5]运城学院物理与电子工程系,运城044000

出　　处：《核技术》2023年第4期71-87,共17页Nuclear Techniques

基　　金：国家重点研发计划(No.2022YFA1604900);国家自然科学基金(No.12275102)资助。

摘　　要：为了从相对论重离子碰撞实验确定量子色动力学(Quantum Chromo-dynamics,QCD)所预言的相变临界点和相边界,必须考虑实验数据中非临界涨落、有限系统尺度、有限演化时间的影响。本文综述了这三方面工作的主要内容、结果和意义。对于非临界涨落,主要讨论了由于有限事件数对观察量测量的影响,估计了在相对论重离子对撞机(Relativistic Heavy-Ion Collider,RHIC)能量扫描区,精确测量高阶守恒荷高阶矩所需要的事件数。提出用泊松分布描述有限末态粒子数所致的统计涨落,将统计涨落和实验结果比较,发现统计涨落贡献为主,必须扣除泊松主导的统计涨落。提出混合事件方法,定义动力学累积矩为原始样本的累积矩减去混合样本的累积矩,利用多相输运模型(A Multiphase Transport Model,AMPT)的default模型重建了一个与之相对应的混合事件样本,结果表明:动力学累积矩确实能很好地扣除泊松样的统计涨落,尤其是中心度bin宽度和探测器效率的影响。对于有限系统尺度的影响,利用三维三态Potts模型研究了各种有限系统尺度下,它的磁化强度的高阶感应率在一级相变、临界点,以及平滑过渡区域的行为。发现在固定外场,穿越相边界的时候,从二阶到六阶磁化率都会出现非单调行为,或符号的变化,而且在三个相变区域,非单调行为类似。因此,仅从非单调行为不能区分不同级数的相变。进一步研究了磁化率的有限尺度标度行为,它们的标度指数在不同级数相变中是不一样的,可以区分不同级数的相变。根据观测量的有限尺度标度性,给出了用固定点确定临界参数的定量方法,并将该方法应用到三维三态Potts模型模拟产生的数据分析,展示了方法的精确有效性。对于非平衡演化的影响,采用Metropolis算法模拟了三维Ising模型在临界点附近从非平衡到平衡的演化过程。发现其序参量在演变过The goal of relativistic heavy-ion collisions is to determine the phase boundary of quantum chromodynamics(QCD)phase transitions.Critically sensitive observables are suggested to be higher-order cumulants of conserved charges.The non-monotonous behavior of higher cumulants was observed at the relativistic heavy-ion collider(RHIC).However,it remains unclear whether these non-monotonous behaviors are critically related.We studied the influences of non-critical fluctuations,finite system size,and limited evolution time to determine if they cause non-monotonous behavior.First,we examined the minimum statistics required for measuring the fourth cumulant.The minimum statistic obtained using the centrality bin width correction(CBWC)method was 25 M.We suggest using a 0.1%centrality bin in the CBWC method instead of each Nch.With a 0.1 centrality bin width,1 M statistics are sufficient.We then pointed out the statistical fluctuations from the limited number of final particles.By assuming the independent emission of each positive(or negative)charged particle,the statistical fluctuations of positive(or negative)charged particles were presented by a Poisson distribution,and the statistical fluctuations of net-charged particles were their evolution.The obtained statistical fluctuations for net protons,net electronic charges,and net baryons were consistent with those from the Hadron Resonance Gas model.In addition,the measured cumulants at RHIC/STAR are dominated by these Poisson-like statistical fluctuations.At the end of this section,we suggest the pooling method of mixed events and demonstrate that the sample of mixed events accurately presents the contributions of the background.Dynamic cumulants were defined as the cumulant of the original sample minus that of the mixed sample.Dynamical cumulants were shown to simultaneously reduce the influence of the statistical fluctuations,centrality bin width effects,and detector efficiency.Second,because the system is finite,the correlation length at the critical point is not develo

关 键 词：相对论重离子碰撞 QCD相变 非临界涨落 有限系统尺度 非平衡 

分 类 号：TL61[核科学技术—核技术及应用] TL612