Decay of

The little curly electron shows that negative particles turn to the right - so the magnetic field is into the screen / paper.

The interaction is between a beam particle entering from below and a proton in the CERN 2m hydrogen bubble chamber.

The 'vee' is caused by the decay of a to .

(Notice: The proton track is slightly darker - hence slower - than the positive track from the primary collision - which, being more curved, has a lower momentum. So the latter is probably a . For it to be a - starting with a beam - would be unlikely from strangeness conservation.)