Phenomenology of Maximal and Near-Maximal Lepton Mixing

Abstract

We study the phenomenological consequences of maximal and near-maximal mixing of the electron neutrino with other ($x$=tau and/or muon) neutrinos. We describe the deviations from maximal mixing in terms of a parameter $ε\equiv1-2\sin^2θ_{ex}$ and quantify the present experimental status for $|ε|<0.3$. We find that the global analysis of solar neutrino data allows maximal mixing with confidence level better than 99% for $10^{-8}$ eV$^2\lsimΔm^2\lsim2\times10^{-7}$ eV$^2$. In the mass ranges $Δm^2\gsim 1.5\times10^{-5}$ eV$^2$ and $4\times10^{-10}$ eV$^2\lsimΔm^2\lsim2\times10^{-7}$ eV$^2$ the full interval $|ε|<0.3$ is allowed within 4$σ$(99.995 % CL). We suggest ways to measure $ε$ in future experiments. The observable that is most sensitive to $ε$ is the rate [NC]/[CC] in combination with the Day-Night asymmetry in the SNO detector. With theoretical and statistical uncertainties, the expected accuracy after 5 years is $Δε\sim 0.07$. We also discuss the effects of maximal and near-maximal $ν_e$-mixing in atmospheric neutrinos, supernova neutrinos, and neutrinoless double beta decay.