# PHENIX measurements of low momentum direct photon radiation

## TL;DR

The PHENIX experiment measured low momentum direct photons across various collision systems at RHIC, revealing excess photon production in A+A and p+A collisions and a universal scaling with charged-particle multiplicity.

## Contribution

This study provides the first comprehensive measurements of low momentum direct photons in small and large collision systems, uncovering universal scaling behaviors.

## Key findings

- Large excess of direct photons in A+A collisions.
- Non-zero excess in central p+A collisions.
- Photon yield scales faster than charged-particle multiplicity.

## Abstract

The versatility of RHIC allowed the PHENIX collaboration to measure low momentum direct photons from small systems, such as p+p, p+A, d+Au at $\sqrt{s_{NN}} = $200 GeV as well as from large A+A systems, such as Au+Au and Cu+Cu at 200 GeV and Au+Au at 62.4 GeV and 39 GeV. In these measurements PHENIX has discovered a large excess over the scaled p+p yield of direct photons in A+A collisions, and a non-zero excess over the scaled p+p yield in central p+A collisions. Another PHENIX discovery is that at low-$p_{T}$ the integrated yield of direct photons, $dN_{\gamma}/dy$, from large systems follows a universal scaling as a function of the charged-particle multiplicity, $(dN_{ch}/d\eta)^{\alpha}$, with $\alpha = 1.25$. The observed scaling properties of direct photons from these systems show that the photon production yield increases faster than the charged-particle multiplicity.