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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A neutrino is a subatomic particle that is part of the Standard Model of particle physics. It is electrically neutral, has an incredibly small mass (which is still not precisely measured but is known to be much less than that of an electron), and interacts very weakly with other matter, meaning it can pass through ordinary matter almost undetected.
First observed directly by the Cowan-Reines neutrino experiment.