Vortex lines and surfaces in positron-atom ionization collisions

We study the appearance of quantum vortices in the ionization of atoms by positron impact. We avoid the standard restrictions on the final-state variables, in order to study vortices as submanifolds of codimension 2 in the multidimensional space of definition of the T-matrix element. In particular we demonstrate that three isolated vortices previously observed under a collinear geometry, actually correspond to a two-dimensional cut on a single vortex line. (paper)