Neutrino Properties

Thanks to very strong evidence that neutrinos oscillate [1], we now know that they almost certainly have masses and mix. That neutrinos have masses means that there is some spectrum of three or more neutrino mass eigenstates, ν1, ν2, ν3, . . ., which are the neutrino analogues of the charged-lepton mass eigenstates, e, μ, and τ . That neutrinos mix means that the weak interaction couples a given charged-lepton mass eigenstate l (= e, μ, or τ) to more than one neutrino mass eigenstate νm. For example, in the weak process W → lνm with a given l, the produced neutrino can be any of the mass eigenstates. The amplitude for it to be the specific mass eigenstate νm, U ∗ lm, is an element of the unitary leptonic mixing matrix, often referred to as the Maki-Nakagawa-Sakata [2] matrix. Since W → lνm with given l yields the neutrino mass eigenstate νm with amplitude U ∗ lm, the neutrino state |νl〉 created in this process is