Dalitz pair production (in a simple 'charge exchange' reaction)

This picture was taken in 1965 in the North Western University 50 cm liquid He bubble chamber, exposed to a beam with momentum of about 180 MeV/c. The beam is entering from the bottom of the picture.

The event in the middle of the picture is interpreted as the 'charge exchange' reaction. The outgoing short track is a low energy proton that stops in the bubble chamber liquid. The decays into two photons, one of which escapes from the bubble chamber while the other 'converts internally' and appears in the bubble chamber as an pair coming from the collision point. The from this pair of tracks spirals spectacularly.

(The pair of tracks produced immediately in this manner happens in about 1% of cases and is known as a Dalitz pair.)