The two rather faint pairs in this picture originate from the same point, where a decayed into two photons .
From measurements of the curvatures of the electron and positron tracks it is possible to estimate their momenta (click here), and hence the momenta of the individual photons . One can then calculate the 'effective mass’ of these two photons. For this event this equals the mass of the . For details, click here.
This picture is one of the most famous bubble chamber pictures in the history of particle physics – the discovery of the omega- .
For details, click here.
For another example of neutral pion decay click on the picture below.