Highlighted is a small part of a complicated high energy
neutrino event produced in the Fermilab 15-foot bubble chamber
filled with a neon hydrogen mixture.
A positron (red) emerging from an electron-positron pair, produced
by a gamma ray, curves round through about 180 degrees. Then it
seems to change charge: it begins to curve in the opposite
direction (blue).
What has happened is that the positron has run head-on into a
(more-or-less from the point of view of particle physics)
stationary electron - transferring all its momentum.
This tells us that the mass of the positron equals the mass
of the electron.
A qualitative appreciation of this can be obtained by
considering the head-on collision of one snooker ball with another:
professionals are able to bring the `projectile' ball to a halt.
(Discuss what would happen if the projectile were made of (a)
polystyrene, and (b) lead.)
Further details can be found in A Lot can happen in a few Billionths
of a Second and A Simple Estimate of the Mass of a Positron.