Collectivity in small collision systems : an initial state perspective

TL;DR

This paper reviews recent developments in understanding collectivity in small collision systems, emphasizing initial state effects as an alternative to hydrodynamic explanations for observed multi-particle correlations.

Contribution

It provides a comprehensive overview of initial state perspectives on collectivity in small systems, highlighting recent progress and unresolved issues.

Findings

Initial state models can reproduce key correlation features.

Hydrodynamic interpretations are challenged by initial state explanations.

Outstanding questions remain about the dominant mechanisms.

Abstract

Measurements of multi-particle correlations in the collisions of small systems such as $p+p$, $p/d/^3He+A$ show striking similarity to the observations in heavy ion collisions. A number of observables measured in the high multiplicity events of these systems resemble features that are attributed to collectivity driven by hydrodynamics. However alternative explanations based on initial state dynamics are able to describe many characteristic features of these measurements. In this brief review we highlight some of the recent developments and outstanding issues in this direction.