Mu-Tau Reflection Symmetry in Lepton Mixing and Neutrino Oscillations

We examine the possibility that the lepton mixing matrix may exhibit a strong form of mu-tau universality, in which corresponding elements of the mu- and tau-neutrino flavour eigenstates have equal moduli, and find that it is consistent with present data on neutrino oscillations. We point out that in the Dirac case, this is equivalent to symmetry under mu-tau reflection, ie. the combined operation of mu-tau flavour exchange in the MNS matrix with a CP transformation on the whole leptonic sector. We give the most general form for such a mixing matrix, examine the lepton mass matrices under such a symmetry, and explore the observable manifestations of the symmetry in neutrino oscillations.