Off-shell Distorted-Wave theory for positronium formation collisions

The formation of positronium in collisions of positrons with atoms is studied by means of different Distorted-Wave descriptions. We observe that even though some of these models manage to describe the available experimental data on an arbitrary scale, they do not show the same degree of consistency in relation to the absolute magnitude, missing the measured total cross sections by one order of magnitude or even more. Here, we analyze the origin of this failure and conclude that an accurate representation of the virtual intermediate states is necessary for a consistent Distorted-Wave theory of Ps formation. Finally, we propose a prescription to deal with this normalization problem.