Positrons are the antimatter version of electrons and so their fate in a matter world is ultimately to annihilate. However, prior to this, a positron may combine with an electron to form a matter-antimatter hybrid called positronium. This is akin to a hydrogen atom with the proton replaced by a positron. Fundamental to our understanding of the physical universe, positron and positronium are these days also acknowledged as being fantastically useful in practical applications such as probing material properties and medical diagnostics. However, there is still much that we do not know for sure about the details of the interactions of these particles with ordinary matter. For example if, in a collision with an atom or molecule, a positron captures an electron, in which directions is the positronium likely to travel and with what probability?
Now Gaetana Laricchia and her team at UCL (Shipman et al PRL 2015 in press) have performed measurements which can, for the first time, be directly compared with theoretical predictions, mostly in disagreement with one another. This is a substantial advance and solves a critical outstanding problem in the field, finally providing an experimental test of a considerable body of theoretical work developed on the subject over the past 40 years. The work is expected to impact also on the resolution of persistent discrepancies in the broader description of charge-exchange. The ratio of the differential and integrated cross-sections for the targets investigated (namely He, Ar, H2 and CO2) exposes the higher propensity for forward-emission of positronium formed from He and H2.