This picture, taken in the Fermilab 15-foot bubble chamber filled with a mixture of neon and hydrogen (Why Ne and H? Click here), and exposed to a high energy neutrino beam, is part of an electromagnetic shower produced in a neutrino interaction. In this picture, the neutrino beam, which produces no bubbles, comes in from the left.
The lone Compton electron near the middle of the picture shows that negative particles turn to the right.
The first highlighted track, which passes close to this Compton electron, is a positron from an pair. It comes to a sudden end near the fiducial mark. At this point the positron has annihilated in flight with an electron, producing a pair of photons, gamma . One of these 'materialises’ in the field of a nucleus, producing an pair, about 11 cm (in the actual bubble chamber) from the annihilation point.
The second highlighted positron also comes from an pair, but this one does not annihilate. Instead, the track - mysteriously, mid-flight - starts curving in the opposite direction, as if it had suddenly become negatively charged. What has happened is that the has run head-on into an electron, transferring all its momentum to that electron . This can only happen if the mass of the positron is equal to that of the electron. (The positron that stops would eventually have annihilated, but there is no evidence of the resulting photons in this picture.)
This is a good picture to show that quantum phenomena are statistical in nature.
If you wondered what the pairs of parallel
lines are, click here.