Collectivity in small systems: Initial correlations or final state flow?

Abstract

I review recent progress in understanding correlation measurements in small collision systems, such as proton+lead and proton+proton collisions at the Large Hadron Collider (LHC) and proton+gold, deuteron+gold, and 3He+gold collisions at the Relativistic Heavy Ion Collider (RHIC). I discuss two distinct theoretical approaches to describing the experimental data on multi-particle correlations. The first attributes the origin of the measured correlations to strong final state interactions, often described by hydrodynamics, the second employs the color glass condensate effective theory and is able to reproduce many features of the data from initial state effects only. I discuss how to distinguish which of the two sources of correlations dominates the experimental observables, and give an outlook on how to make progress on the theory side.