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arXiv 1111.6538nucl-thhep-ph
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重离子碰撞中流体动力学响应对初始几何的映射

Mapping the hydrodynamic response to the initial geometry in heavy-ion collisions

Fernando G. Gardim, Frederique Grassi, Matthew Luzum, Jean-Yves Ollitrault

备注：
Version accepted for publication (minor changes); 7 pages, 4 figues; Figure data files included with source
发表信息：
Phys. Rev. C 85, 024908 (2012)
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摘要：

我们研究了如何将重离子碰撞的初始几何形状转化为最终的流动观测值，通过求解具有实际波动初始条件的逐事件理想流体力学。本文定量地研究了一组实际模拟的各向异性流（v_n）在多大程度上受初始偏心率的影响，并讨论了初始偏心率的定义对v_n的最佳估计。我们发现，在定义varepn时使用r^2权重的常见做法通常会导致v_n的预测值比使用r^n权重时更差，n > 2。我们同样研究了初始状态的附加性质的重要性。例如，我们证明，为了正确预测非中心碰撞的v_4和v_5，必须考虑分别与（ε_2）^2和（ε_2）*（ε_3）成比例的非线性项。我们发现，无论是在一个快度范围内计算偏心率，还是在z=0的单个切片中计算偏心率，都没有区别，使用能量或熵密度权重也不重要。这方面的知识将是重要的，使实验观测值和流体动力学初始条件之间的更直接的联系，后者是目前约束不足。

英文摘要：

We investigate how the initial geometry of a heavy-ion collision is transformed into final flow observables by solving event-by-event ideal hydrodynamics with realistic fluctuating initial conditions. We study quantitatively to what extent anisotropic flow (v_n) is determined by the initial eccentricity epsilon_n for a set of realistic simulations, and we discuss which definition of epsilon_n gives the best estimator of v_n. We find that the common practice of using an r^2 weight in the definition of varepsilon_n in general results in a poorer predictor of v_n than when using r^n weight, for n > 2. We similarly study the importance of additional properties of the initial state. For example, we show that in order to correctly predict v_4 and v_5 for non-central collisions, one must take into account nonlinear terms proportional to (epsilon_2)^2 and (epsilon_2)*(epsilon_3), respectively. We find that it makes no difference whether one calculates the eccentricities over a range of rapidity, or in a single slice at z=0, nor is it important whether one uses an energy or entropy density weight. This knowledge will be important for making a more direct link between experimental observables and hydrodynamic initial conditions, the latter being poorly constrained at present.

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