Cronin effect and high – p ⊥ suppression in the nuclear gluon distribution at small x

We present a systematic, and fully analytic, study of the ratio RpA between the gluon distribution in a nucleus and that in a proton scaled up by the atomic number A. We consider initial conditions of the McLerran–Venugopalan type, and quantum evolution in the Color Glass Condensate, with both fixed and running coupling. We perform an analytic study of the Cronin effect in the initial conditions and point out an interesting difference between saturating effects and twist effects in the nuclear gluon distribution. We show that the distribution of the gluons which make up the condensate in the initial conditions is localized at low momenta, but this particularity does not survive after the quantum evolution. We demonstrate that the rapid suppression of the ratio RpA in the early stages of the evolution is due to the DGLAP–like evolution of the proton, whose gluon distribution grows much faster than that in the nucleus because of the large separation between the respective saturation momenta. The flattening of the Cronin peak, on the other hand, is due to the evolution of the nucleus. We emphasize interesting differences between the fixed coupling and the running coupling scenarios, which are particulary important in the late stages of the evolution. Preprint submitted to Elsevier Science 5 March 2008