Abstract
We review recent developments in the theoretical description and understanding of multi-particle correlation measurements in collisions of small projectiles (p/d/3He) with heavy nuclei (Au, Pb) as well as proton+proton collisions. We focus on whether the physical processes responsible for the observed long range rapidity correlations and their azimuthal structure are the same in small systems as in heavy ion collisions. In the latter they are interpreted as generated by the initial spatial geometry being transformed into momentum correlations by strong final state interactions. However, explicit calculations show that also initial state momentum correlations are present and could contribute to observables in small systems. If strong final state interactions are present in small systems, recent developments show that results are sensitive to the shape of the proton and its fluctuations.
Highlights
Multi-particle correlation measurements in collisions of small projectiles (e.g. p/d/3He) with other small projectiles or heavy ions (e.g. Au, Pb) show characteristic structures that are long range in rapidity and have azimuthal anisotropies very similar to what was measured in heavy ion collisions [1]
Most calculations concerned with momentum correlations that are intrinsic to the initially produced particle distributions are based on the color glass condensate (CGC) effective theory of quantum chromo dynamics (QCD) [45]
Whether the initial state framework can describe such trends remains to be seen. If it is the case the origin of the change in v3 for example must be different than in the final state framework. This is because it has been shown [58, 79] that there is no correlation between the global event geometry and the produced momentum anisotropy in the initial state picture
Summary
Multi-particle correlation measurements in collisions of small projectiles (e.g. p/d/3He) with other small projectiles or heavy ions (e.g. Au, Pb) show characteristic structures that are long range in rapidity and have azimuthal anisotropies very similar to what was measured in heavy ion collisions [1]. In heavy ion collisions these structures have for a long time been interpreted as emerging from the system’s response (via strong final state interactions) to the initial shape of the interaction region [2, 3, 4, 5]. In this case the long range nature of the correlation is due to the transverse geometry being almost rapidity independent. In the following we discuss the question of applicability of hydrodynamics, the role of subnucleonic fluctuations, and the contribution from initial state momentum correlations. We follow with a discussion of what observables could give further insight into what source of correlation dominates in a given system for a given multiplicity range
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