Geometrical Scaling of Direct Photons in Relativistic Heavy Ion and d+Au Collisions

Abstract

In this paper, we show that multiplicity spectra of direct photons in A+A and d+Au collisions at different centrality classes and different energies exhibit geometrical scaling, {\em i.e.}, they depend on a specific combination of number of participants $N_{\rm part}$, collisions energy $W$, and transverse momentum $p_{T}$ -- called saturation scale -- rather than on all these three variables separately. In particular, the dependence on the geometry of collisions encoded in the dependence on $N_{\rm part}$ is in agreement with the expectations based on the Color Glass Condensate theory.