Vortices in ionization collisions by positron impact

The presence of vortices in the ionization of hydrogen atoms by positrons at intermediate impact energies is investigated. The present findings show that a previously reported minimum in the fully differential cross section is the signature of a vortex in the continuum positron-electron-proton system. The behaviour of the real and imaginary parts of the complex-valued transition matrix is studied in order to determine and characterize the vortex in momentum space. The obtained information is translated to fully differential ionization cross sections, feasible of being measured with currently available techniques.