This event shows an interaction of an 8.2 GeV/c particle with a proton in the CERN 2 metre hydrogen bubble chamber.
The small curly tracks, curving anticlockwise, that can be seen on some beam tracks are electrons which have been given a small amount of energy by the beam particle. From their motion, it can be seen that the magnetic field is out of the page (towards the reader).
The two outgoing charged particles are a (dark !) and a . The 'vee' pointing back to the interaction is produced when a neutral produced in the original collision decays to a and a .
Measurements show that energy and momentum are conserved, so there are no missing particles. The reaction is
For details of how to use momentum measurements to identify the neutral decaying particle (vee) as a K0 click here.