Phenomenology of maximal and near-maximal lepton mixing

The possible existence of maximal or near-maximal lepton mixing constitutes an intriguing challenge for fundamental theories of flavor. 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≡1-2sin²θ_ex and quantify the present experimental status for |ε|<0.3. We show that both probabilities and observables depend on ε quadratically when effects are due to vacuum oscillations and they depend on ε linearly if matter effects dominate. The most important information on ν_e mixing comes from solar neutrino experiments. We find that the global analysis of solar neutrino data allows maximal mixing with confidence level better than 99% for 10^-8 eV²≲Δm²≲2×10^-7 eV². In the mass ranges Δm²≳1.5×10^-5 eV² and 4×10^-10 eV²≲Δm²≲2×10^-7 eV² the full interval |ε|<0.3 is allowed within ∼4σ (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∼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.