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, i.e., they depend on a specific combination of number of participants Npart, collisions energy W, and transverse momentum pT – 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 Npart is in agreement with the expectations based on the Color Glass Condensate theory.