"Figure 4 (b)" of "Beam energy dependence of moments of the net-charge multiplicity distributions in Au+Au collisions at RHIC"

We report the first measurements of the moments - mean ($M$), variance ($\sigma^{2}$), skewness ($S$) and kurtosis ($\kappa$) - of the net-charge multiplicity distributions at mid-rapidity in Au+Au collisions at seven energies, ranging from $\sqrt{s_{NN}}=$ 7.7 to 200~GeV, as a part of the Beam Energy Scan program at RHIC. The moments are related to the thermodynamic susceptibilities of net-charge, and are sensitive to the location of the QCD critical point. We compare the products of the moments, $\sigma^{2}/M$, $S\sigma$ and $\kappa\sigma^{2}$ with the expectations from Poisson and negative binomial distributions (NBD). The $S\sigma$ values deviate from Poisson and are close to NBD baseline, while the $\kappa\sigma^{2}$ values tend to lie between the two. Within the present uncertainties, our data do not show non-monotonic behavior as a function of collision energy. These measurements provide a valuable tool to extract the freeze-out parameters in heavy-ion collisions by comparing with theoretical models.

本研究首次报道了相对论重离子对撞机（RHIC）束流能量扫描计划中，7.7~200 GeV核子-核子质心系能量（$sqrt{s_{NN}}$）范围内金-金（Au+Au）碰撞快度中点处净电荷多重数分布的各阶矩——均值（$M$）、方差（$sigma^2$）、偏度（$S$）与峰度（$kappa$）的测量结果。这些矩与净电荷的热力学磁化率相关，且对量子色动力学（Quantum Chromodynamics, QCD）临界点的位置具有敏感性。我们将各阶矩的组合量$sigma^2/M$、$Ssigma$与$kappasigma^2$与泊松分布及负二项分布（NBD）的理论预期进行了对比。其中$Ssigma$的测量值偏离泊松分布结果，且接近负二项分布的基准预期；而$kappasigma^2$的测量值则介于两者之间。在当前的误差范围内，我们的数据未呈现出随碰撞能量变化的非单调行为。通过与理论模型对比，本测量结果为提取重离子碰撞中的冻出参数提供了极具价值的研究手段。