Limits on the diffractive mass in strong coherent ${\gamma }^{\ast }$-nucleus scattering

An evolution equation for diffractive production with a definite rapidity gap is given. Coherent ${\gamma }^{\ast }A$ collisions in the unitarity (saturation) region are studied with the conclusion that the diffractive mass is always on the order of the ${\gamma }^{\ast }$ virtuality, $Q$, when the scattering is strong. Deep in the saturation region diffractive masses significantly greater than $Q$ are strongly suppressed by a Levin-Tuchin mechanism.