小系统中的十年集体性
A Decade of Collectivity in Small Systems
AI总结：
本文综述了过去十年在pp、p-Pb及ep、e+e-等小碰撞系统中发现的集体性现象，评估了现有动力学模型的理论基础及其随系统尺寸的标度行为，并澄清了现有数据对所有动力学解释的挑战。
摘要：
长期以来，在（超）相对论核-核碰撞中观察到了集体性的特征，包括方位各向异性流、径向流以及特征强子化学依赖性。这些特征支撑了在解释这些碰撞系统时关于QGP形成和近完美流体性的解释。然而，值得注意的是，在过去十年中，所有这些集体性特征基本上都在小至pp和p-Pb的碰撞系统中被识别出来，而传统上认为这些系统中不存在集体现象。甚至在ep和e+e-碰撞中也可能发现了前兆现象。本文提供了关于小系统集体性所有数据的完整综述。在可能的情况下，它回顾了这些数据的模型模拟。然而，由于缺乏对所有系统尺寸集体性的现象学完全令人满意的描述，我们特别关注重离子碰撞中所有集体性动力学框架的理论基础，以及它们随系统尺寸的预期标度行为。我们的讨论澄清了所有动力学解释在多大程度上受到现有数据的挑战。
英文摘要：
Signatures of collectivity, including azimuthally anisotropic and radial flow as well as characteristic hadrochemical dependencies, have been observed since long in (ultra)relativistic nucleus-nucleus collisions. They underpin the interpretation of these collision systems in terms of QGP formation and close-to-perfect fluidity. Remarkably, however, essentially all these signatures of collectivity have been identified within the last decade in collision systems as small as pp and p-Pb, where collective phenomena had been assumed to be absent traditionally. Precursor phenomena may have been found even in ep and $e^+e^-$ collisions. This article provides a complete review of all data on small system collectivity. It reviews model simulations of these data where available. However, in the absence of a phenomenologically fully satisfactory description of collectivity across all system sizes, we focus in particular on the theoretical basis of all dynamical frameworks of collectivity invoked in heavy ion collisions, and their expected scaling with system size. Our discussion clarifies to what extent all dynamical explanations are challenged by the available data.
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