Large Solar Neutrino Mixing and Radiative Neutrino Mechanism

Abstract

We find that the presence of a global $L_e-L_{\mu }-L_{\tau }$ ($\equiv L^{\prime }$) symmetry and an $S_2$ permutation symmetry for the $\mu$- and $\tau$-families supplemented by a discrete $Z_4$ symmetry naturally leads to almost maximal atmospheric neutrino mixing and large solar neutrino mixing, which arise, respectively, from type II seesaw mechanism initiated by an $S_2$-symmetric triplet Higgs scalar $s$ with $L^{\prime }=2$ and from radiative mechanism of the Zee type initiated by two singly charged scalars, an $S_2$-symmetric $h^{+}$ with $L^{\prime }=0$ and an $S_2$-antisymmetric $h^{\prime +}$ with $L^{\prime }=2$. The almost maximal mixing for atmospheric neutrinos is explained by the appearance of the democratic coupling of $s$ to neutrinos ensured by $S_2$ and $Z_4$ while the large mixing for solar neutrinos is explained by the similarity of $h^{+}$- and $h^{\prime +}$-couplings described by $f_{+}^h\sim f_{-}^h$ and ${\mu }_{+}\sim {\mu }_{-}$, where $f_{+}^h$ ($f_{-}^h$) and ${\mu }_{+}$ (${\mu }_{-}$) stand for $h^{+}$ ($h^{\prime +}$)-couplings, respectively, to leptons and to Higgs scalars.

Comments: RevTex, 10 pages including one figure. In Ref.[25], the cited page number is corrected