muonneutrino

A muon neutrino (νμ) is one of the three known flavors of neutrinos, elementary leptons that interact only via the weak force and gravity. As a flavor eigenstate, νμ is associated with the muon and carries lepton number Lμ = +1. In the Standard Model, neutrinos have very small but nonzero masses and mix with each other, so a νμ produced in one process is a superposition of mass eigenstates.

νμs are produced in copious numbers in the decays of charged pions and kaons, such as π+ → μ+ νμ, and in muon decay μ− → e− ν̄e νμ. They are also created in cosmic-ray interactions in the atmosphere and in accelerator beams for neutrino experiments. Neutrinos interact with matter predominantly via the weak interaction; charged-current interactions with W bosons convert a νμ into a muon, while neutral-current interactions with Z bosons leave the flavor unchanged.

Detection relies on observing the products of these interactions. In charged-current events a muon is produced and leaves a long track in Cherenkov or scintillation detectors, enabling momentum determination and directional information. Experiments such as Super-Kamiokande, IceCube, MINOS, T2K, and NOvA study νμ fluxes across a range of energies.

Neutrino oscillations cause νμ to change flavor as they propagate, most notably into ντ, and also into νe. The effect is described by the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix, with parameters including the mixing angle θ23 and the mass-squared difference Δm^2_32. Atmospheric and accelerator experiments have established substantial νμ disappearance and provided measurements of these parameters; the ordering of neutrino masses and potential CP violation in the lepton sector remain active areas of research.