  We use a diagrammatic approach to study color-neutral heavy particle
production in nucleus-nucleus collisions in a quasi-classical approximation
without small-$x$ evolution. In order to treat the two nuclei symmetrically, we
use the Coulomb gauge which gives the appropriate light cone gauge for each
nucleus. The resulting cross section is factorized into a product of two
Weizs\"{a}cker-Williams gluon distributions of the two nuclei when the
transverse momentum of the produced scalar particle is around the saturation
momentum. We confirm our results in covariant gauge where the transverse
momentum broadening of hard gluons can be described as a diffusion process. The
transverse momentum factorization manifests itself in light cone gauge but not
so clearly in covariant gauge.
