lepton number, a special quantum number that characterizes leptons. Experiment has shown that in all processes the difference between the numbers of leptons and the numbers of their antiparticles remains constant. For example, the absorption of an electron (e–) by a proton (p) in the process of nuclear K-capture is accompanied by the emergence of an electron neutrino (νe), e– + p → n + νe, and the absorption of a negative muon (μ–) is accompanied by the emergence of a muon neutrino (vμ), μ– + p→ n + νμ; in the process of beta decay of a neutron (n) an electron antineutrino (v̄e) is produced together with an electron; and so on. This principle may be explained by assuming that leptons have a special charge, the lepton charge L, which is conserved in processes involving the conversion of elementary particles and has opposite signs for particles and antiparticles.
Current experimental data indicate the existence of two lepton charges, the electron charge Le and the muon charge Lμ (since the total number of electrons and electron neutrinos and the total number of muons and muon neutrinos are conserved separately). Usually the following values are used: Le= +1 for e– and ve; Le = — 1 for e+ and v̄e; Lμ = +1 for μ– and vμ; and Lμ= —1 for μ,+ and v̄μ. (However, the experimental data also may be explained by assuming the existence of a single lepton charge L that assumes opposite values for e– and μ–; that is, L = + 1 for e-, ve, μ+, and v̄μ and L = — 1 for e +, v̄e, μ–, and vμ.) For all other elementary particles the lepton charge is assumed to be equal to zero.
The lepton charge of a system of particles is equal to the algebraic sum of the lepton charges of the constituent particles, and thus the law of conservation of the number of leptons reduces to the law of conservation of lepton charge. (Similarly, the law of conservation of the number of baryons reduces to the law of conservation of baryon charge.)
The lepton charge, in contrast to electric charge, is not a source of any long-range field. However, it is possible that the role of the lepton charge in elementary particle physics has not yet been fully elucidated.
S. S. GERSHTEIN