  The presence of vortices in the ionisation of hydrogen atoms by positrons at
intermediate impact energies is investigated. The present findings show that a
previously reported minima 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 ionisation
cross sections, feasible of being measured with currently available techniques.
