A study on the isolated photon production in nuclear collisions at the CERN-LHC energies

Abstract

An analysis of prompt photon production in high energy nuclear collisions at the Large Hadron Collider is performed within the parton saturation picture taking into account the updated phenomenological color dipole models. Comparison between ⟨N coll⟩ scaling for hard scattering in heavy-ion collisions and the N part-scaling based on geometric scaling arguments has been done. The predictions are parameter free in the first case whereas a dependence on the constant of proportionality κ between the number of participants and the nuclear saturation scale appears in the second case. This parameter has been analyzed in the prompt photon spectrum at small transverse momentum even though no fitting procedure was performed. Results are confronted with the measurements made by the ALICE, ATLAS, and CMS experiments in terms of photon transverse momentum at different rapidity bins. We show that the prompt photon production exhibits distinct scalings in AA events associated to geometrical properties of the collision and can be properly addressed in the color dipole formalism. Based on the N part-scaling, an analytical parametrization for the invariant cross section is provided and employed to predict the x T-scaling in measurements. For κ of order of unit the theoretical scaling curve correctly describes data in the range x T ⩽ 5 × 10−2.