Eur. Phys. J. C (2020) 80: 670
https://doi.org/10.1140/epjc/s10052-020-8219-9

Regular Article - Theoretical Physics

Scaling properties of direct photon yields in heavy ion collisions

¹ Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York, 11794-3800, USA
² Institute of Theoretical Physics, Jagiellonian University, S. Łojasiewicza 11, 30-348, Kraków, Poland

^* e-mail: michal@if.uj.edu.pl

Received: 23 April 2020
Accepted: 7 July 2020
Published online: 24 July 2020

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 -spectra for transverse momenta up to 2 GeV/c are scaled with charged hadron pseudorapidity density at midrapidity raised to power . On the other hand, those direct photon -spectra also exhibit geometrical scaling in the similar range. Assuming power-law dependence of the scaled photon spectra for both scaling laws, we formulate two independent conditions for the power , which overshoot experimental data by on average. We discuss possible sources that might improve this estimate.