Lepton number is a conserved quantum number in particle physics that is used to describe the number of leptons in a system. Leptons are a family of elementary particles that include charged leptons (such as electrons, muons, and tau particles) and neutral leptons (such as neutrinos). The lepton number is defined as follows: - Each lepton (such as an electron or a neutrino) has a lepton number of +1.
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Lepton number is an additive label assigning to leptons, to antileptons and zero to quarks. Ordinary muon decay conserves total lepton number: the final electron and muon neutrino contribute two positive units while the electron antineutrino contributes one negative unit.
Separate electron-, muon- and tau-flavor numbers are not individually conserved by neutrino propagation with mixing. This does not by itself establish violation of total lepton number. A Dirac neutrino mass can preserve the total label, whereas a Majorana mass term changes it by two units in the usual assignment. Total lepton number is therefore a useful bookkeeping rule whose exact status depends on the specified interactions, not an identity that must hold in every extension of the theory.