  Ionization collisions have important consequences in many physical phenomena,
and the mechanism that leads to ionization is not universal. Double
differential cross sections (DDCSs) are often used to identify ionization
mechanisms because they exhibit features that distinguish close collisions from
grazing collisions. In the angular DDCS, a sharp peak indicates ionization
through a close binary collision, while a broad angular distribution points to
a grazing collision. In the DDCS energy spectrum, electrons ejected through a
binary encounter collision result in peak at an energy predicted from momentum
conservation. These insights into ionization processes are well-established for
plane wave projectiles. However, the recent development of sculpted particle
wave packets reopens the question of how ionization occurs for these new
particle wave forms. We present theoretical DDCSs for (e,2e) ionization of
atomic hydrogen for electron vortex projectiles. Our results predict that the
ionization mechanism for vortex projectiles is similar to that of non-vortex
projectiles, but that features in the DDCS that distinguish ionization
mechanisms are obscured by the projectile's momentum uncertainty. Additionally,
the projectile's non-zero transverse momentum increases the cross section for
high energy ejected electrons.
