# Direct real photons in relativistic heavy ion collisions

## TL;DR

This review discusses the role of direct real photons as penetrating probes in relativistic heavy ion collisions, highlighting experimental and theoretical advances and addressing the unresolved 'direct photon puzzle' related to photon yields and asymmetries.

## Contribution

It provides a comprehensive overview of developments since the 1970s, emphasizing recent challenges like the direct photon puzzle in heavy ion collision studies.

## Key findings

- Large photon yields observed in experiments
- Strong azimuthal asymmetries detected in photon emissions
- The 'direct photon puzzle' remains unresolved

## Abstract

Direct real photons are arguably the most versatile tools to study relativistic heavy ion collisions. They are produced, by various mechanisms, during the entire space-time history of the strongly interacting system. Also, being colorless, most the time they escape without further interaction, ie they are penetrating probes. This makes them rich in information, but hard to decypher and interpret. This review presents the experimental and theoretical developments related to direct real photons since the 1970s, with a special emphasis on the recently emerged "direct photon puzzle", the simultaneous presence of large yields and strong azimuthal asymmetries of photons in heavy ion collisions, an observation that so far eluded full and coherent explanation.