High energy pA collisions in the color glass condensate approach I: gluon production and the Cronin effect

Abstract

We study gluon production in high energy proton–nucleus collisions in the semi-classical framework of the color glass condensate. We develop a general formalism to compute gluon fields in covariant gauge to lowest order in the classical field of the proton and to all orders in the classical field of the nucleus. The use of the covariant gauge makes the diagrammatic interpretation of the solution more transparent. k ⊥ -factorization holds to this order for gluon production—our results for the gluon distribution are equivalent to the prior diagrammatic analysis of Kovchegov and Mueller. We also show that these results are equivalent to the computation of gluon production by Dumitru and McLerran in the Fock–Schwinger gauge. We demonstrate how the Cronin effect arises in this approach, and examine its behavior in the two extreme limits of (a) no small- x quantum evolution, and (b) fully saturated quantum evolution. In both cases, the formalism reduces to Glauber's formalism of multiple scatterings. We comment on the possible implications of this study for the interpretation of the recent results on deuteron–gold collisions at the Relativistic Heavy Ion Collider (RHIC).