# Scaling properties of direct photon yields in heavy ion collisions

## TL;DR

This paper analyzes the scaling behaviors of direct photon yields in heavy ion collisions across various systems and energies, revealing universal multiplicity and geometrical scaling patterns in photon spectra.

## Contribution

It introduces a formulation of two independent conditions for the scaling power laws in direct photon spectra, comparing them with experimental data.

## Key findings

- Photon spectra exhibit universal multiplicity scaling with charged hadron density.
- Photon spectra also show geometrical scaling in the same momentum range.
- The formulated power-law conditions overshoot experimental data by about 10%.

## Abstract

A recent analysis from the PHENIX collaboration of available direct photon measurement results in collisions of various systems such as Au+Au, Cu+Cu, and Pb+Pb, at different beam energies ranging from 39 to 2760 GeV, has shown a universal, within experimental uncertainties, $multiplicity$ scaling, in which direct photon $p_{T}$-spectra for transverse momenta up to 2 GeV/$c$ are scaled with charged hadron pseudorapidity density at midrapidity raised to power $\alpha=1.25$. On the other hand, those direct photon $p_{T}$-spectra also exhibit $geometrical$ scaling in the similar $p_{T}$ range. Assuming power-law dependence of the scaled photon spectra for both scaling laws, we formulate two independent conditions for the power $\alpha$, which overshoot experimental data by $\sim 10\%$ on average. We discuss possible sources that might improve this estimate.