II.2.3 Lepton Symmetry
In any interaction between elementary particles, the lepton number represents the difference between the number of leptons and the number of anti-leptons, and it is a preserved quantity. Additionally, separate lepton family numbers are defined for the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino. For example, the creation of an electron is accompanied by the creation of an electron antineutrino, and the creation of a positron is accompanied by the creation of an electron neutrino. Likewise, a decaying negative muon results in the creation of a muon neutrino, while a decaying positive muon results in the creation of a muon antineutrino.
Lepton flavor is only approximately conserved, and is notably not conserved in neutrino oscillation. However, total lepton number is still conserved in the Standard Model. Numerous searches for physics beyond the Standard Model incorporate searches for lepton number or lepton flavor violation. Because the lepton number conservation law in fact is violated by chiral anomalies, there are problems applying this symmetry universally over all energy scales.