Collectivity in small collision systems : an initial state perspective

Understand

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.