  We propose a partonic picture for high-mass diffractive dissociation events
in onium-nucleus scattering, which leads to simple and robust predictions for
the distribution of the sizes of gaps in diffractive dissociation of virtual
photons off nuclei at very high energies. We show that the obtained probability
distribution can formally be identified to the distribution of the decay time
of the most recent common ancestor of a set of objects generated near the edge
of a branching random walk, and explain the physical origin of this appealing
correspondence. We then use the fact that the diffractive cross section
conditioned to a minimum rapidity gap size obeys a set of Balitsky-Kovchegov
equations in order to test numerically our analytical predictions. Furthermore,
we show how simulations in the framework of a Monte Carlo implementation of the
QCD evolution support our picture.
