Lepton Decays

The heavier lepton types (muon and tau) are not found in ordinary matter because they decay extremely quickly to lighter leptons and sometimes quarks and antiquarks. Electrons and neutrinos are the only stable leptons.

When a heavy lepton decays, one of its resulting particles is always its matching neutrino. The other resulting particles can be any lepton plus its antineutrino or alternatively any quark and its antiquark.

If we assign electron number +1 to electrons and electron-type neutrinos, -1 to positrons and electron-type antineutrinos, and zero to all other particles, then electron number is conserved in all processes. Similarly muon number and tau number can be defined.

For example, consider the decay of the muon:

Leptons Quark Interactions