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 (cursive chi=tau and/or muon) neutrinos. We describe the deviations from maximal mixing in terms of a parameter ∈≡1-2 sin² θ_{ecursive chi} and quantify the present experimental status for |∈|<0.3. We show that both probabilities and observables depend on ∈ quadratically when effects are due to vacuum oscillations and they depend on ∈ 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≳1.510^-5 eV² and 4 × 10^-10 eV²≲Δm²≲2×10^-7 eV² the full interval |∈|<0.3 is allowed within ∼4σ (99.995% CL) We suggest ways to measure ∈ in future experiments. The observable that is most sensitive to ∈ 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 Δ∈∼0.07. We also discuss the effects of maximal and near-maximal ν_e mixing in atmospheric neutrinos, supernova neutrinos, and neutrino-less double beta decay.