The muon (from the letter mu (μ)--used to represent it) is an elementary particle with negative electric charge and a spin of 1/2. It has a mean lifetime of 2.2?s, longer than any other unstable lepton, meson or baryon except for the neutron. Together with the electron, the tau, and the neutrinos, it is classified as a lepton. Like all fundamental particles, the muon has an antimatter partner of opposite charge but equal mass and spin: the antimuon, also called a positive muon. Muons are