Dubna/Serpukhov: Rare decays

When physicists talk about 'rare decays', they frequently mean decays which are forbidden by conventional selection rules. However a sighting of any such forbidden decay would immediately be indicative of new physics, and the search continues. However there are examples of allowed processes which are genuinely rare, for example when pairs of produced particles unite into atom-like states, as in the decay of a neutral pion into a photon and a positronium atom (electronpositron bound state) observed at the Serpukhov proton synchrotron in a collaboration between the Joint Institute for Nuclear Research (JINR, Dubna) and the Institute for High Energy Physics