Scaling properties of direct photon yields in heavy ion collisions

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.

8 pages, 5 figures. The paper has been rewritten, title has been changed, new material and references added. Version accepted for publication in EPJ C. In v3 references updated, minor typos corrected