Scaling properties of direct photon yields in heavy ion collisions

Abstract

AbstractA 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.