Lepton number

Lepton number is an additive quantum number assigned +1 to leptons, −1 to antileptons, and 0 to particles outside those classes. It is conserved in many interactions, though not as an exact symmetry in all known physics.

Lepton flavor number: Separate additive quantum numbers assigned to electron, muon, and tau leptons and their antiparticles. Flavor-specific conservation refines total lepton-number bookkeeping in particle reactions.
Lepton: An elementary fermion that does not participate in the strong interaction. Lepton number is assigned to this particle family and its antiparticles.
Wolfgang Pauli: An Austrian-born theoretical physicist who formulated the neutrino hypothesis in 1930. His proposed neutral particle helped account for missing energy in beta decay.
Baryogenesis: The set of processes that produced the observed excess of matter over antimatter in the universe. Lepton-number-violating processes can contribute to matter generation through leptogenesis.
Neutrinoless double beta decay: A hypothetical nuclear decay that emits two electrons without emitting neutrinos. Its observation would demonstrate violation of total lepton number by two units.
Neutrino: An electrically neutral elementary lepton that interacts through the weak force and gravity. Neutrinos carry lepton number despite having no electric charge.
Enrico Fermi: An Italian-American physicist who developed the first theory of beta decay. His theory made neutrinos central to particle-reaction bookkeeping.
Leptogenesis: A proposed explanation of the cosmic matter excess through early-universe lepton asymmetry converted into baryon asymmetry. It uses lepton-number violation to help explain why matter dominates antimatter.
Majorana fermion: A fermion that is its own antiparticle. A Majorana neutrino can permit processes that violate lepton number.
Antiparticle: A particle counterpart with the same mass and opposite values of additive charges. Antileptons receive the opposite lepton number from their corresponding leptons.

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