Radiatively Generated Neutrino Masses in SU(3)_L x U(1)_N Gauge Models

In SU(3)_L x U(1)_N gauge models for electroweak interactions, we discuss how to implement a radiative mechanism of generating Majorana neutrino masses by considering the property that the Higgs scalar, which has a coupling to a charged lepton (\ell)-neutrino (\nu) pair, is naturally included. The mechanism is shown to work in models with a heavy charged lepton, \omega^+, in a lepton triplet (\nu, \ell, \omega^+) and with a heavy neutral lepton, N, in (\nu, \ell, N). A minimal model with \ell and \ell^c in (\nu, \ell, \ell^c) together with a sextet Higgs scalar suffers from a fine-tuning problem to suppress tree-level neutrino masses.Comment: 6 pages by RevTex and 1 figure (with a reference added

LMA MSW Solution in the Minimal SU(3)L×U(1)NSU(3)_L \times U(1)_N Gauge Model

The minimal $SU(3)_L \times U(1)_N$ gauge model for $(\nu,\ell,\ell^C)$ equipped with the (approximate) $L_e-L_\mu-L_\tau$ symmetry and a discrete $Z_4$ symmetry is found to provide radiative neutrino masses compatible with the LMA MSW solution.Comment: Talk given by T.K. at the International Workshop NuFACT'01, Tsukuba, Japan (May 2001

What Does mu-tau Symmetry Imply about Neutrino Mixings?

The requirement of the mu-tau symmetry in the neutrino sector that yields the maximal atmospheric neutrino mixing is shown to yield either sin(\theta_{13})=0 (referred to as C1)) or sin(\theta_{12})=0 (referred to as C2)), where \theta_{12(13)} stands for the solar (reactor) neutrino mixing angle. We study general properties possessed by approximately mu-tau symmetric textures. It is argued that the tiny mu-tau symmetry breaking generally leads to cos(2\theta_{23}) \simsin(\theta_{13}) for C1) and cos(2\theta_{23}) \sim \Delta m^2_\odot/\Delta m^2_{atm}(\equiv R) for C2), which indicates that the smallness of cos(2\theta_{23}) is a good measure of the mu-tau symmetry breaking, where \Delta m^2_{atm} (\Delta m^2_\odot) stands for the square mass differences of atmospheric (solar) neutrinos. We further find that the relation R \sim sin^2(\theta_{13}) arises from contributions of O(sin^2(\theta_{13})) in the estimation of the neutrino masses (m_{1,2,3}) for C1), and that possible forms of textures are strongly restricted to realize sin^2(2\theta_{12})=O(1) for C2). To satisfy R \sim sin^2(\theta_{13}) for C1), neutrinos exhibit the inverted mass hierarchy, or the quasi degenerate mass pattern with | m_{1,2,3}| \sim O(\sqrt{\Delta m^2_{atm}}), and, to realize sin^2(2\theta_{12})=O(1) for C2), there should be an additional small parameter \eta whose size is comparable to that of the mu-tau symmetry breaking parameter \epsilon, giving tan(2\theta_{12}) \sim \epsilon/\eta with \eta \sim \epsilon to be compatible with the observed large mixing.Comment: 10 pages, title slightly modified, comments added in the introdction, typos corrected, references updated, version to appear in Physical Reviews