lepton (lĕpˈtŏnˌ) [key] [Gr., = light (i.e., lightweight)], class of elementary particles that includes the electron and its antiparticle, the muon and its antiparticle, the tau and its antiparticle, and the neutrino and antineutrino associated with each of these particles. Leptons are the lightest class of particles having nonzero rest mass. From a technical point of view, they are defined by their behavior, being weakly interacting fermions, i.e., leptons can result from the slow decay of nuclear particles such as the neutron but do not experience a strong attraction toward the nuclear particles; they are described by the Fermi-Dirac statistics, which apply to all particles restricted by the Pauli exclusion principle. This means that two identical leptons cannot occupy the same quantum state. However, one muon and one electron are allowed to occupy the same state. The muon was originally classed as a meson because of its mass, about 200 times that of the electron, but the subsequent reclassification of particles on the basis of their behavior placed it with the electron in the lepton category. The electron and the muon are almost twins, except for their large mass difference; each is negatively charged, has a positively charged antiparticle, and has an associated neutrino and antineutrino. Separate laws govern the conservation of electron family number and of muon family number, the number being +1 for ordinary particles of either family and - 1 for antiparticles (see conservation laws, in physics).
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