CLVisc流体力学中各向异性流的赝快度分布和去相关性
Pseudorapidity distribution and decorrelation of anisotropic flow within CLVisc hydrodynamics
摘要：
软强子和硬和电磁探针的密度和强相互作用介质的波动和相关性的研究需要事件的高能重离子碰撞的计算密集型事件的流体动力学模拟。我们开发了一个（3+1）维粘性流体动力学模型CLVisc，并使用开放计算语言（OpenCL）在图形处理器（GPU）上并行化，相对于没有GPU并行化的模型，时空演化的性能提高了60倍，Cooper-Frye粒子化的性能提高了120倍以上。然后在CLVisc中计算了各向异性流$v_n(η)$的伪快度依赖性，初始条件由A多相输运（AMPT）模型给出，能量密度在横向和纵向上都有波动。虽然$v_n(η)$的大小以及$v_2(η)$和$v_3(η)$之间的比率对有效剪切粘度与熵密度比率$η_v/s$敏感，但$η$中的$v_{n}(η)$分布的形状不依赖于$η_v/s$的值。本文还研究了由于初始部分子密度分布中事件面的扭曲和起伏引起的$v_n$沿赝快度方向的去相关。当没有初始流体速度时，发现具有辅助参考窗口$η^b$的$v_n\{-η^a\}$和$v_n\{η^a\}$之间的去相关可观测$r_n(η^a, η^b)$对$η_v/s$不敏感。对于小的$η_v/s$，来自微喷流部分子的初始流体速度在两个参考快度窗口$η^b \in [3, 4]$和$η^b \in [4.4, 5.0]$之间引入了相当大的$r_n(η^a, η^b)$分裂，正如在实验中观察到的那样。还提供了CLVisc的实现以及如何在GPU上高效并行化科学程序的指导方针。
英文摘要：
Studies of fluctuations and correlations of soft hadrons and hard and electromagnetic probes of the dense and strongly interacting medium require event-by-event hydrodynamic simulations of high-energy heavy-ion collisions that are computing intensive. We develop a (3+1)D viscous hydrodynamic model -- CLVisc that is parallelized on Graphics Processing Unit (GPU) using Open Computing Language (OpenCL) with 60 times performance increase for space-time evolution and more than 120 times for the Cooper-Frye particlization relative to that without GPU parallelization. The pseudo-rapidity dependence of anisotropic flow $v_n(η)$ are then computed in CLVisc with initial conditions given by the A Multi-Phase Transport (AMPT) model, with energy density fluctuations both in the transverse plane and along the longitudinal direction. Although the magnitude of $v_n(η)$ and the ratios between $v_2(η)$ and $v_3(η)$ are sensitive to the effective shear viscosity over entropy density ratio $η_v/s$, the shape of the $v_{n}(η)$ distributions in $η$ do not depend on the value of $η_v/s$. The decorrelation of $v_n$ along the pseudo-rapidity direction due to the twist and fluctuation of the event-planes in the initial parton density distributions is also studied. The decorrelation observable $r_n(η^a, η^b)$ between $v_n\{-η^a\}$ and $v_n\{η^a\}$ with the auxiliary reference window $η^b$ is found not sensitive to $η_v/s$ when there is no initial fluid velocity. For small $η_v/s$, the initial fluid velocity from mini-jet partons introduces sizable splitting of $r_n(η^a, η^b)$ between the two reference rapidity windows $η^b \in [3, 4]$ and $η^b \in [4.4, 5.0]$, as has been observed in experiment. The implementation of CLVisc and guidelines on how to efficiently parallelize scientific programs on GPUs are also provided.
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