Positronium

Positronium is a state related to an electron and its antiparticle, the positron. It was first observed in 1951 by the physicist Martin Deutsch.

Its study has recently returned to the fore because of its implications as a test of physics beyond the standard model.

Positronium is unstable and disintegrates in less than 200 nanoseconds, generally into two gamma photons, but not always.