Scaling properties of direct photon yields in heavy ion collisions

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 $$p_{T}$$pT-spectra for transverse momenta up to 2 GeV/c are scaled with charged hadron pseudorapidity density at midrapidity raised to power $$\alpha =1.25$$α=1.25. On the other hand, those direct photon $$p_{T}$$pT-spectra also exhibit geometrical scaling in the similar $$p_{T}$$pT range. Assuming power-law dependence of the scaled photon spectra for both scaling laws, we formulate two independent conditions for the power $$\alpha $$α, which overshoot experimental data by $$\sim 10\%$$∼10% on average. We discuss possible sources that might improve this estimate.