Lepton number assigns to leptons, to antileptons and zero to quarks. Neutrino masses show that separate family lepton numbers need not be exact.
A neutrino is an electrically neutral spin-one-half lepton. The Standard Model has three active neutrino flavours associated with the electron, muon and tau.
If all neutrinos are exactly massless, arbitrary unitary rotations among them preserve both their kinetic terms and their zero mass matrix. After diagonalizing the charged-lepton Yukawa matrix, choose the neutrino basis to coincide with that rotation. The resulting weak charged current has no physical lepton mixing matrix. With nondegenerate neutrino masses this extra basis freedom is lost, permitting observable neutrino oscillations and lepton mixing.
The muon neutrino is the neutral lepton coupled to a muon by the weak charged current. It carries lepton number ; its antineutrino has the opposite lepton number.
Neutrino oscillation is the coherent conversion between neutrino flavors caused by different propagation phases of mixed mass eigenstates. It measures mass-squared differences and mixing parameters.
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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.