Understanding how momentum anisotropies arise in small collision systems is important for a quantitative understanding of collectivity in terms of QCD dynamics in small and large collision systems. In this letter we present results for small collision systems from the newly developed parton cascade Alpaca, which faithfully encodes the AMY effective kinetic theory. Alpaca reproduces quantitatively previously known results from a calculation in the single-hit approximation for small values of the coupling. We discuss in detail how such a comparison is to be carried out. Particularly at larger coupling a generic difference between the two approaches becomes apparent, namely that in parton cascades particles interact over a finite distance while in direct integrations of the Boltzmann equation the interactions are local. This leads to quantitative differences in the extracted values for the elliptic flow coefficient. These discrepancies appear in situations where the mean free path is not large compared to the interaction time and the applicability of kinetic theory is thus questionable.

中文翻译：

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小系统和 AMY parton 级联羊驼中的单击近似


了解小型碰撞系统中动量各向异性如何产生，对于定量理解小型和大型碰撞系统中 QCD 动力学的集体性非常重要。在这封信中，我们介绍了新开发的 parton 级联羊驼的小型碰撞系统的结果，它忠实地编码了 AMY 有效动力学理论。Alpaca 定量地再现了先前已知的已知结果，该结果来自耦合的小值的单次命中近似计算。我们将详细讨论如何进行此类比较。特别是在较大的耦合中，两种方法之间的一般差异变得明显，即在 parton 级联中，粒子在有限距离上相互作用，而在玻尔兹曼方程的直接积分中，相互作用是局部的。这会导致椭圆流动系数的提取值存在定量差异。这些差异出现在平均自由程与相互作用时间相比不大的情况下，因此动力学理论的适用性值得怀疑。