A lepton is a fundamental particle of matter that does not take part in strong interactions. All leptons are fermions and have spin ½. The charged leptons are the electron, the muon, the tau lepton, and their antiparticles. Neutral leptons are called neutrinos. Lepton number is the total number of leptons present in a system minus the total number of antileptons. Lepton conservation is a rule which states that the net number of leptons before and after an interaction must be the same. The name 'lepton' comes from Greek leptos meaning 'small' or 'slight' and was coined by L. Rosenfeld in 1948.
The muon is the second lightest lepton. It has an electric charge of -1. It is identical to the electron but is 207 times more massive. It decays with a mean lifetime of 2.2 × 10-6 sec. Muons occur commonly in cosmic rays. Despite its name, a muon (short for "mu meson") is no longer classified as a meson.
On learning of the discovery of the muon, the physicist Isidor Isaac Rabi said: "Who ordered that?"
Muonium is a short-lived entity formed by a muon and its antiparticle attracted together and revolving about a common center.
The tau lepton is the third charged lepton (in order of increasing mass), with an electric charge of -1, a spin of ½, and a mass of 1.784 GeV. It is unstable with a lifetime of 3 × 10-13 sec. The tau lepton was first observed in 1975 in experiments carried out at SLAC (Stanford Linear Accelerator), having already been predicted in theory.
Lepton flavour violation
Lepton flavor violation is the principle, in the Standard Model, that leptons do not change flavor. For example, a muon would never turn into an electron. If this were observed, it would be a signal for new physics. Evidence for neutrino oscillations already indicate that lepton flavor violation may occur.