The heavier leptons, the muon and the tau,
are not found in ordinary matter at all. This is
because when they are produced they very quickly decay, or transform, into lighter leptons.
Sometimes the tau lepton will decay into a quark, an antiquark, and a tau neutrino. Electrons and the three kinds of
neutrinos are stable and thus the types we commonly see around us.
When a heavy lepton decays, one of the particles it decays into is always its corresponding
neutrino. The other particles could be a quark and its antiquark, or another lepton and its
Physicists have observed that some types of
lepton decays are possible and some are not. In order to explain this, they divided the
leptons into three lepton families: the electron and its neutrino, the muon and its neutrino, and
the tau and its neutrino. The
number of members in each family must remain constant in a decay.
(A particle and an antiparticle in the same family "cancel out"
to make the total of them equal zero.)
Although leptons are solitary, they are always loyal to their families!