Collectivity in ultra-peripheral heavy-ion and e+A collisions

TL;DR

This paper reviews recent theoretical advances in understanding collective phenomena in photon+nucleus collisions, focusing on initial state correlations and hydrodynamic responses to collision geometry.

Contribution

It synthesizes different theoretical frameworks, including color glass condensate and hydrodynamics, to explain collective effects in ultra-peripheral heavy-ion and e+A collisions.

Findings

Color glass condensate encodes initial state correlations.

Hydrodynamics explains final state momentum correlations.

Unified view of initial and final state effects in collisions.

Abstract

We review recent theoretical progress in describing collective effects in photon+nucleus collisions. The approaches considered range from the color glass condensate where correlations are encoded in the initial state, to hydrodynamic frameworks, where a strong final state response to the initial geometry of the collision is the key ingredient to generate momentum-space correlations.