重离子碰撞中定向流响应的驱动与阻尼
Driver and damping of the directed-flow response in heavy-ion collisions
- Panchayat College（潘查亚特学院）
- Universidad de Tarapacá（塔拉帕卡大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
本研究通过改进的AMPT模型区分重离子碰撞中初始几何驱动与介质耗散对定向流的影响，发现集体行为阈值质量数约为35，并提取出定向流阻尼尺度Kn0≈0.27及η/s在0.10-0.20之间。
AI中文摘要：
初始态几何在驱动定向流中起着关键作用，而介质的耗散响应则对其产生阻尼。这两个因素共同影响定向流斜率随系统尺寸的变化方式。我们开发了一种方法，利用改进版的弦熔化AMPT模型，在$\u221as_{\mathrm{NN}} = 200$~GeV的O+O、Cu+Cu、Ru+Ru、Au+Au和U+U碰撞中，区分这些驱动和阻尼效应对带电强子定向流的影响。我们基于熵密度、参与者数和质量数构建了三个标度观测量。构造了一个无量纲比率，从熵和参与者标度中揭示了中心碰撞中的阈值质量数$A \approx 35$，表明集体行为的开始。我们构建了一个动力学理论克努森数（$\mathrm{Kn}$）图，以分析初始态驱动（随$\mathrm{Kn}^{\kappa}$增长，$\kappa \approx 2$）和最终态粘性阻尼（具有特征尺度$\mathrm{Kn}_0 \approx 0.27$的流体动力学响应）对定向流斜率的贡献。发现该阻尼尺度比从椭圆流中提取的$\mathrm{Kn}_0 \approx 0.7$约小2.5倍。此外，我们使用一种不依赖拟合流谐波的替代方法，确定了剪切粘度与熵密度之比$\eta/s$在0.10到0.20之间。
英文摘要：
The initial-state geometry plays a crucial role in driving directed flow, while the dissipative response of the medium dampens it. Both of these factors influence how the directed-flow slope varies with system size. We developed a method to differentiate between these driving and damping effects on charged-hadron directed flow in O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U collisions at $\sqrt{s_{\mathrm{NN}}} = 200$~GeV using an improved version of the string-melting AMPT model. We formulated three scaling observables based on the entropy density, the number of participants, and the mass number. A dimensionless ratio was constructed, revealing the threshold mass number $A \approx 35$ in central collisions from the entropy and participant scaling, indicating the onset of collective behavior. We constructed a kinetic-theory Knudsen-number ($\mathrm{Kn}$) map to analyze the contributions of the initial-state driver, which grows as $\mathrm{Kn}^κ$ with $κ\approx 2$, and a final-state viscous damping of the hydrodynamic response with characteristic scale $\mathrm{Kn}_0 \approx 0.27$ for the directed flow slope. This damping scale is found to be about a factor of 2.5 smaller than the $\mathrm{Kn}_0 \approx 0.7$ extracted from the elliptic flow. Furthermore, we determined the ratio of shear viscosity to entropy density, $η/s$, to be between 0.10 and 0.20, using an alternative method that does not rely on fitting flow harmonics.
