Texas A&M University, College Station, Texas 77843.
Czech Technical University in Prague, FNSPE, Prague 115 19, Czech Republic.
Phys Rev Lett. 2023 Feb 24;130(8):082301. doi: 10.1103/PhysRevLett.130.082301.
We report the beam energy and collision centrality dependence of fifth and sixth order cumulants (C_{5}, C_{6}) and factorial cumulants (κ_{5}, κ_{6}) of net-proton and proton number distributions, from center-of-mass energy (sqrt[s_{NN}]) 3 GeV to 200 GeV Au+Au collisions at RHIC. Cumulant ratios of net-proton (taken as proxy for net-baryon) distributions generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at 3 GeV. The measured values of C_{6}/C_{2} for 0%-40% centrality collisions show progressively negative trend with decreasing energy, while it is positive for the lowest energy studied. These observed negative signs are consistent with QCD calculations (for baryon chemical potential, μ_{B}≤110 MeV) which contains the crossover transition range. In addition, for energies above 7.7 GeV, the measured proton κ_{n}, within uncertainties, does not support the two-component (Poisson+binomial) shape of proton number distributions that would be expected from a first-order phase transition. Taken in combination, the hyperorder proton number fluctuations suggest that the structure of QCD matter at high baryon density, μ_{B}∼750 MeV at sqrt[s_{NN}]=3 GeV is starkly different from those at vanishing μ_{B}∼24 MeV at sqrt[s_{NN}]=200 GeV and higher collision energies.
我们报道了质心能量和碰撞中心度对净质子和质子数分布的第五阶和第六阶累积量（C_{5}，C_{6}）以及阶乘累积量（κ_{5}，κ_{6}）的依赖关系，这些数据来自于 RHIC 上的 3GeV 到 200GeV 的 Au+Au 碰撞。净质子（作为净重子的代理）分布的累积量比通常遵循 QCD 热力学预期的层次结构，除了在 3GeV 碰撞的情况下。在 0%-40%中心度碰撞中，测量得到的 C_{6}/C_{2}比值随着能量的降低呈现出逐渐负向的趋势，而在研究的最低能量下则为正值。这些观测到的负号与 QCD 计算结果一致（对于重子化学势，μ_{B}≤110 MeV），其中包含了交叉转换范围。此外，在 7.7GeV 以上的能量下，测量得到的质子 κ_{n}，在不确定性范围内，不支持质子数分布的两分量（泊松+二项式）形状，这是从一级相变中预期的。综合来看，超阶质子数涨落表明，在高重子密度（μ_{B}∼750 MeV，在 sqrt[s_{NN}]=3 GeV）下 QCD 物质的结构与在零重子密度（μ_{B}∼24 MeV，在 sqrt[s_{NN}]=200 GeV 及更高的碰撞能量下）的结构明显不同。
