[论文解读] Collective phenomena in the early stages of relativistic heavy-ion collisions

Radosław Ryblewski|arXiv (Cornell University)|Jul 3, 2012

High-Energy Particle Collisions Research参考文献 6被引用 3

一句话总结

该论文提出ADHYDRO，一种用于相对论重离子碰撞的新型流体动力学模型，将理想流体动力学推广至描述物质形成早期阶段中强各向异性和耗散系统的模型。通过引入广义状态方程和动态熵源项，ADHYDRO成功再现了RHIC（200 GeV）处的软强子可观测量，揭示了各向异性相持续约1 fm，并证实了集体流在初始各向异性下仍具有鲁棒性。

ABSTRACT

A recently developed framework of highly-anisotropic and strongly-dissipative hydrodynamics -- ADHYDRO -- has been introduced and used to analyze the space-time evolution of matter produced in ultra-relativistic heavy-ion collisions. The main goal of this analysis was to study the effect of initial highly-anisotropic stages on the final soft hadronic observables typically measured in the experiment. The study was done in the context of the heavy-ion measurements performed at RHIC (Relativistic Heavy Ion Collider) in Brookhaven National Laboratory. Starting from the general assumption about the form of the phase-space distribution function, thermodynamic properties of locally anisotropic systems of particles have been studied and the form of the generalized equation of state has been formulated. The dynamic equations determining the evolution of a highly-anisotropic fluid have been introduced. The form of the entropy source related to the mechanisms leading to thermalization of the system has been defined. In the simplest case of purely-longitudinal and boost-invariant expansion, different features of the model have been analyzed. Using the ADHYDRO model in the general (3+1)D and boost-invariant (2+1)D versions, different possible scenarios of early stages of heavy-ion collisions have been analyzed. The results of the ADHYDRO model have been compared to the results obtained from the reference, perfect-fluid hydrodynamic model -- LHYQUID. The results of the hydrodynamic models were coupled to the statistical Monte-Carlo model THERMINATOR. The following conclusions have been drawn: a) All studied observables are almost insensitive to the initial anisotropic stage provided the initial conditions of the evolution are properly readjusted, b) Complete thermalization of matter may take place only at the times of about 1 fm/c. In this way the early thermalization puzzle may be circumvented.

研究动机与目标

• 开发一种能够描述相对论重离子碰撞中早期强各向异性物质的流体动力学框架。
• 在一致的热力学与流体动力学形式体系中，纳入非平衡效应，如压力各向异性和耗散。
• 在不同初始各向异性条件下，检验集体流可观测量（如$v_2$、$v_1$、HBT半径）的鲁棒性。
• 通过与RHIC实验数据拟合，确定各向异性相的持续时间。
• 在双ustinvariant与非invariant情形下，验证模型对软强子谱和关联函数的描述能力。

提出的方法

• 提出ADHYDRO，一种基于广义相空间分布函数的高各向异性和强耗散流体动力学模型，其中包含各向异性的温度参数$\lambda_\perp$、$\lambda_\parallel$。
• 推导出一种广义状态方程，可在各向异性和各向同性区域之间平滑插值，实现连续的各向同性化。
• 从输运理论中导出动态熵源项，通过弛豫函数$R(x)$建模热化过程。
• 采用Cooper-Frye冻结-out形式，从演化中的流体动力学超曲面提取物理可观测量。
• 在1+1D双ustinvariant和完整3+1D非invariant配置下应用该模型，以匹配实验数据。
• 将ADHYDRO与THERMINATOR模型结合，实现统计强子化，计算末态粒子谱和关联函数。

实验结果

研究问题

• RQ1 包含强初始各向异性和耗散的流体动力学模型能否准确描述RHIC处的软强子可观测量？
• RQ2 集体流可观测量（$v_2$、$v_1$）和HBT半径对初始动量空间各向异性的敏感性如何？
• RQ3 根据实验数据约束，重离子碰撞早期阶段的各向异性相持续时间是多少？
• RQ4 该模型是否能再现观测到的横动量流速普适性，无论初始各向异性如何？
• RQ5 包含非双ustinvariant初始条件如何影响对非中央碰撞的描述？

主要发现

• ADHYDRO模型成功描述了在$\sqrt{s_{\rm NN}} = 200$ GeV下，软强子区域（$p_T < 3$ GeV）的单粒子谱、$v_2$、$v_1$和HBT半径。
• 在中心快度区，可观测量对初始各向异性参数的敏感性较弱，支持横动量流速的普适性。
• 通过与实验数据拟合，估计各向异性相持续约1 fm，与微观物理模型一致。
• 在小各向异性极限下，ADHYDRO退化为Israel-Stewart理论，证实其与标准黏性流体动力学的一致性。
• 即使在无双ustinvariance条件下，模型仍与实验数据保持良好一致，表明非中央碰撞中集体流的鲁棒性。
• 从输运理论导出的熵源项使热化动力学在早期演化中实现自洽描述。