– 1– Neutrino Physics as Explored by Flavor Change

I. The physics of flavor change: The rather convincing evidence that atmospheric neutrinos change from one flavor to another has now been joined by new, very strong evidence that the solar neutrinos do this as well. Neutrino flavor change implies that neutrinos have nonzero masses. That is, there is a spectrum of three or more neutrino mass eigenstates, that are the analogues of the charged-lepton mass eigenstates, e, µ, and τ. Neutrino flavor change also implies leptonic mixing. That is, the weak interaction coupling the W boson to a charged lepton and a neutrino can couple any charged-lepton mass eigenstate α to any neutrino mass eigenstate ν i. Here, α = e, µ, or τ, and e is the electron, etc. Leptonic W + decay can yield a particular + α in association with any ν i. The amplitude for this decay to produce the specific combination + α + ν i is U * αi , where U is the unitary leptonic mixing matrix [1]. Thus, the neutrino state created in the decay W + → + α + ν is the state |ν α = i U * αi |ν i. This superposition of neutrino mass eigenstates, produced in association with the charged lepton of " flavor " α, is the state we refer to as the neutrino of flavor α. While there are only three (known) charged lepton mass eigenstates, the experimental results suggest that perhaps there are more than three neutrino mass eigenstates. If, for example, there are four ν i , then one linear combination of them, |ν s = i U * si |ν i , (2) does not have a charged-lepton partner, and consequently does not couple to the Standard Model W boson. Indeed, since the decays Z → ν α ν α of the Standard Model Z boson have been found to yield only three distinct neutrinos ν α of definite flavor [2], ν s does not couple to the Z boson either. Such CITATION: K. Hagiwara et al.