Vortices in the three-body electron–positron–proton continuum system induced by the positron-impact ionization of hydrogen

Abstract

By means of a systematic theoretical study of the transition matrix element T for the ionization collisions in a collinear geometry, we uncover the presence of three isolated zeros at intermediate impact energies. We demonstrate that these zeros actually represent vortices in the generalized velocity field associated to T. One of these vortices is shown to be related to a deep minimum observed more than two decades ago by Brauner and Briggs in 1991 at extremely large impact energies, where the corresponding fully differential cross section was too small to be experimentally accessible. Here we elucidate that it might still be present at much smaller impact energies, thus being amenable to experimental investigation. Furthermore, we discovered that this vortex is paired with a second one of opposite circulation, in accordance with one of the scenarios for their emergence. We study the location of these vortices and find that they seem to be located at, emerge from, or move towards specific points which could be related to particular collision mechanisms, as the direct ionization and capture to the continuum cusps, or the saddle-point and Thomas processes.