PICTURES OF PARTICLE PRODUCTION IN PROTON-NUCLEUS COLLISIONS

This work focuses on gluon (jet) production in dilute (proton)-dense (nucleus) collisions. Depending on the frame and gauge, gluon production can be viewed as a freeing of gluons coming from either the proton wave function or from the nucleus wave function. These (apparently) very different pictures must lead to the same result and the purpose of this paper is to see how that happens. The focus is on gluons having k(perpendicular to) similar to Q(S) or gluons in the scaling region k(perpendicular to)/Q(S) >> 1. In the McLerran-Venugopalan (MV) model with k(perpendicular to) similar to Q(S), we are able to derive gluon production in a way that (graphically) manifestly shows k(perpendicular to)-factorization in terms of the number density of gluons in the nuclear wave function. We presume that this picture, and k(perpendicular to)-factorization, continues to hold in the presence of small-x evolution although we have not been able to explicitly verify this. Our result is in agreement with usual k(perpendicular to)-factorization where the gluon number density of the nucleus does not appear in an explicit way.