Geometrical scaling of direct-photon production in hadron collisions from RHIC to the LHC

Geometric scaling is a property of hadronic interactions predicted by theories of gluon saturation and expressing rates in terms of dimensionless ratios of transverse momentum to the saturation momentum. In this paper we consider production of photons in pp, dAu and AuAu collisions at sNN=200GeV (RHIC) and in PbPb collisions at sNN=2760GeV (LHC) and show that the yield of direct photons in the transverse momentum range 1GeV<pT≤4GeV/c satisfies geometric scaling. Excellent agreement with geometric scaling is obtained with the only free parameter of the saturation momentum determined previously via the dependence of the saturation momentum upon Bjorken x and centrality.