Neutrino Mixing: from the Broken \mu-\tau Symmetry to the Broken Friedberg-Lee Symmetry

Abstract

I argue that the observed flavor structures of leptons and quarks might imply the existence of certain flavor symmetries. The latter should be a good starting point to build realistic models towards deeper understanding of the fermion mass spectra and flavor mixing patterns. The \mu-\tau permutation symmetry serves for such an example to interpret the almost maximal atmospheric neutrino mixing angle (\theta_23 \sim 45^\circ) and the strongly suppressed CHOOZ neutrino mixing angle (\theta_13 < 10^\circ). In this talk I like to highlight a new kind of flavor symmetry, the Friedberg-Lee symmetry, for the effective Majorana neutrino mass operator. Luo and I have shown that this symmetry can be broken in an oblique way, such that the lightest neutrino remains massless but an experimentally-favored neutrino mixing pattern is achievable. We get a novel prediction for \theta_13 in the CP-conserving case: \sin\theta_13 = \tan\theta_12 |(1- \tan\theta_23)/ (1+ \tan\theta_{23})|. Our scenario can simply be generalized to accommodate CP violation and be combined with the seesaw mechanism. Finally I stress the importance of probing possible effects of \mu-\tau symmetry breaking either in terrestrial neutrino oscillation experiments or with ultrahigh-energy cosmic neutrino telescopes.