arXiv 1504.021602015-04-09nucl-th

预平衡演化对重离子碰撞观测量的影响

Pre-equilibrium evolution effects on heavy-ion collision observables

Jia Liu, Chun Shen, Ulrich W. Heinz

摘要：

为了研究预平衡动力学对相对论重离子碰撞观测量的重要性，我们将由MonteCarlo Kharzeev-Levin-Nardi（MC-KLN）和MonteCarlo Glauber（MC-Glb）模型产生的自由流部分子模拟的高度非平衡早期演化阶段与由粘性流体动力学描述的局部近似热化晚期演化阶段相匹配，并研究最终强子横向动量分布的依赖关系，特别是它们的基本径向和各向异性流，在这些阶段之间的切换时间。对π介子、K介子和质子的平均横动量$\langle p_\perp \rangle$以及带电强子的椭圆流和三角流$v_{2,3}^\mathrm{ch}$的测量值进行了三参数拟合，其中转换时间$τ_s$、流体动力学阶段的比剪切粘度$η/s$和动力学退耦温度$T_\mathrm{dec}$作为自由参数，我们发现优选的“热化”时间$τ_s$强烈地依赖于初始条件的模型。与MC-Glb初始条件相比，MC-KLN初始条件需要更早地转变为流体动力学行为（在$τ_s \approx$ 0.13 fm/ $c$），随后是具有更大比剪切粘度$η/s\approx$ 0.2的流体动力学演变，MC-Glb初始条件优选在较晚的时间（$τ_s\approx$ 0.6 fm/ $c$）进行切换，随后是具有$η/s\approx$ 0.16的较低粘度的流体动力学演变。这些新的结果，包括预平衡的演变进行比较，适合没有一个预平衡阶段，所有的动态演变之前，流体动力学行为的发病被忽略。在每种情况下，质量的动态描述的优化参数集，以及表现出最强的约束力的热化时间的观测值，进行了讨论。

英文摘要：

In order to investigate the importance of pre-equilibrium dynamics on relativistic heavy-ion collision observables, we match a highly non-equilibrium early evolution stage, modeled by free-streaming partons generated from the Monte Carlo Kharzeev-Levin-Nardi (MC-KLN) and Monte Carlo Glauber (MC-Glb) models, to a locally approximately thermalized later evolution stage described by viscous hydrodynamics, and study the dependence of final hadronic transverse momentum distributions, in particular their underlying radial and anisotropic flows, on the switching time between these stages. Performing a 3-parameter fit of the measured values for the average transverse momenta $\langle p_\perp \rangle$ for pions, kaons and protons as well as the elliptic and triangular flows of charged hadrons $v_{2,3}^\mathrm{ch}$, with the switching time $τ_s$, the specific shear viscosity $η/s$ during the hydrodynamic stage, and the kinetic decoupling temperature $T_\mathrm{dec}$ as free parameters, we find that the preferred "thermalization" times $τ_s$ depend strongly on the model of the initial conditions. MC-KLN initial conditions require an earlier transition to hydrodynamic behavior (at $τ_s \approx$ 0.13 fm/$c$) , followed by hydrodynamic evolution with a larger specific shear viscosity $η/s\approx$ 0.2, than MC-Glb initial conditions which prefer switching at a later time ($τ_s\approx$ 0.6 fm/$c$) followed by a less viscous hydrodynamic evolution with $η/s\approx$ 0.16. These new results including pre-equilibrium evolution are compared to fits without a pre-equilbrium stage where all dynamic evolution before the onset of hydrodynamic behavior is ignored. In each case, the quality of the dynamical descriptions for the optimized parameter sets, as well as the observables which show the strongest constraining power for the thermalization time, are discussed.

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摘要：

英文摘要：

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