The observation in small size collision systems, pp and pA, of strong correlations with long range in rapidity and a characteristic structure in azimuth, the ridge phenomenon, is one of the most interesting results obtained at the large hadron collider. Earlier observations of these correlations in heavy ion collisions at the relativistic heavy ion collider are standardly attributed to collective flow due to strong final state interactions, described in the framework of viscous relativistic hydrodynamics. Even though data for small size systems are well described in this framework, the applicability of hydrodynamics is less well grounded and initial state based mechanisms have been suggested to explain the ridge. In this review, we discuss particle correlations from the initial state point of view, with focus on the most recent theoretical developments.

中文翻译：

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初始状态下的粒子相关性

pp和p在小型碰撞系统中的观察在大强子对撞机上获得的最有趣的结果之一是A，具有很强的相关性，且具有快速的远距离变化和特定的方位结构（脊现象）。相对论重离子对撞机在重离子碰撞中对这些相关性的早期观察通常归因于归因于粘滞相对论流体力学框架中强大的最终状态相互作用的集体流动。即使在此框架中很好地描述了小型系统的数据，流体力学的适用性也不是很好的基础，并且已经提出了基于初始状态的机制来解释这一难题。在这篇综述中，我们从初始状态的角度讨论粒子相关性，重点是最新的理论发展。