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 $p_{T}$-spectra for transverse momenta up to 2 GeV/$c$ are scaled with charged hadron pseudorapidity density at midrapidity raised to power $α=1.25$. 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 $α$, which overshoot experimental data by $\sim 10\%$ on average. We discuss possible sources that might improve this estimate.

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