A classic example of a pi mu e decay

This picture was taken in the CERN 2m hydrogen bubble chamber. (We think the incoming beam consists of particles at 10 GeV/c.)

The little curly electron near the collision point tells us that negative particles turn to the left.

The track that starts going to the right before looping round is a . It stops and decays to a and a muon neutrino . The muon can only receive about 30 MeV/c (for details click here) in this decay and can only travel about 1 cm in hydrogen before it , itself, stops. It then decays into a positron (which spirals characteristically), an electron neutrino and a muon-antineutrino .

The full decay sequence is

For further examples, click on one of the pictures below.