  We present a partonic picture for diffractive onium-nucleus scattering in the
large-number-of-color limit from which the distribution of rapidity gaps in a
certain kinematic region can be deduced. This picture allows us to draw a
parallel between diffractive dissociation and the genealogy of partonic
evolution, the latter being essentially similar to a branching-diffusion
process in which the branching is the parton splitting, and the diffusion
occurs in the transverse momenta of the partons. In particular, we show that
the rapidity gap distribution corresponds to the distribution of the splitting
rapidity of the last common ancestor of the partons whose transverse momenta
are smaller than the nuclear saturation scale, when the scattering process is
viewed in the restframe of the nucleus. Numerical calculations are also
implemented to support the analytical predictions.
