Centrality dependence of the direct photon multiplicity in heavy ion collisions

TL;DR

This paper provides a theoretical perspective on the centrality dependence of direct photon multiplicity in heavy ion collisions, emphasizing the need for stronger experimental constraints and revisiting theoretical models of photon production.

Contribution

It offers a theoretical analysis of experimental observations and highlights the importance of further experimental and theoretical studies to understand photon production mechanisms.

Findings

Photon multiplicity scales with the number of binary collisions.

Scaling does not depend on the photon multiplicity cutoff.

Revisiting jet-medium and medium-modified prompt photon models is necessary.

Abstract

Measurements indicate that the centrality dependence of the direct photon multiplicity scales approximately with the number of binary nucleon collisions. Importantly, these same measurements suggest that this scaling does not depend on the cutoff used to define the photon multiplicity. In this work we provide a theoretical perspective on these observations. We emphasize the importance of stronger experimental constraints, in particular for the cutoff independence of the centrality scaling. We highlight the importance of revisiting theoretical studies of jet-medium photons and of medium-modified prompt photons to clarify their contribution to the direct photon multiplicity, as well as the magnitude and direction of their momentum anisotropy.