On deviations from bimaximal neutrino mixing

The PMNS neutrino mixing matrix U-PMNS is in general a product of two unitary matrices U-lep and U-nu arising from the diagonalization of the charged lepton and neutrino mass matrices, U-PMNS = U-lep(dagger). Assuming that U-nu is a bimaximal mixing matrix, we investigate the possible forms of U-lep. We identify three possible generic structures of U-lep, which are compatible with the existing data on neutrino mixing. One corresponds to a hierarchical "CKM-like" matrix. In this case relatively large values of the solar neutrino mixing angle theta(sol), and of \U-e3\(2) drop \(U-PMNS)(e3)\(2), are typically predicted, tan(2)theta(sol) greater than or similar to 0.42, \U-e3\(2) greater than or similar to 0.02, while the atmospheric neutrino mixing angle theta(atm) can deviate noticeably from pi/4, sin(2) 2theta(atm) greater than or similar to 0.95. The second corresponds to one of the mixing angles in U-lep being equal to pi/2, and predicts practically maximal atmospheric neutrino mixing sin(2) 2theta(atm) similar or equal to 1. Large atmospheric neutrino mixing, sin2 2theta(atm) greater than or similar to 0.95, is naturally predicted by the third possible generic structure of U-lep, which corresponds to all three mixing angles in U-lep being large. We focus especially on the case of CP-non-conservation, analyzing it in detail. We show how the CP-violating phases, arising from the diagonalization of the neutrino and charged lepton mass matrices, contribute to the measured neutrino mixing observables. (C) 2004 Elsevier B.V. All rights reserved.