Maximal atmospheric neutrino mixing in an SU(5) model

We show that maximal atmospheric and large solar neutrino mixing can be implemented in SU(5) gauge theories, by making use of the $U(1)_F$ symmetry associated with a suitably defined family number $F$, together with a \mathbbm{Z}_2 symmetry which does not commute with $F$. $U(1)_F$ is softly broken by the mass terms of the right-handed neutrino singlets, which are responsible for the seesaw mechanism; in addition, $U(1)_F$ is also spontaneously broken at the electroweak scale. In our scenario, lepton mixing stems exclusively from the right-handed-neutrino Majorana mass matrix, whereas the CKM matrix originates solely in the up-type-quark sector. We show that, despite the non-supersymmetric character of our model, unification of the gauge couplings can be achieved at a scale $10^{16} \mathrm{GeV} < m_U < 10^{19} \mathrm{GeV}$; indeed, we have found a particular solution to this problem which yields results almost identical to the ones of the Minimal Supersymmetric Standard Model.Comment: 17 pages, Latex, no figure

A model for trimaximal lepton mixing

We consider trimaximal lepton mixing, defined by |U_{alpha 2}|^2 = 1/3 for all alpha = e, mu, tau. This corresponds to a two-parameter lepton mixing matrix U. We present a model for the lepton sector in which trimaximal mixing is enforced by softly broken discrete symmetries; one version of the model is based on the group Delta(27). A salient feature of our model is that no vacuum alignment is required.Comment: 14 pages, 1 figure; minor corrections, references added, final version for JHE

Models of maximal atmospheric neutrino mixing and leptogenesis

We discuss two extensions of the Standard Model based on the seesaw mechanism and on non-abelian family symmetry groups O(2) and $D_4$, respectively. Both models have a twofold-degenerate neutrino Dirac mass matrix $M_D$, a Majorana mass matrix invariant under a $\mu$--$\tau$ interchange symmetry and the predictions of maximal atmospheric neutrino mixing and vanishing mixing angle $\theta_{13}$. Leptogenesis can naturally be incorporated if $10^{-3} \lesssim m_1 \lesssim 10^{-2}$ eV where $m_1$ the mass of the lightest neutrino and if the relevant heavy neutrinos are in the range $10^{11}$ to $10^{12}$ GeV. The $D_4$ model is more constrained and leptogenesis requires $m_1$ to be in the vicinity of $4 \times 10^{-3}$ eV.Comment: 8 pages, one figure, talk presented at NOON2004, February 11-15, 2004, Tokyo, Japa

Softly broken lepton numbers and maximal neutrino mixing

We consider lepton mixing in an extension of the Standard Model with three right-handed neutrino singlets. We require that the three lepton numbers L_e, L_\mu, and L_\tau be separately conserved in the Yukawa couplings, and we assume that they are softly broken only by the Majorana mass matrix M_R of the neutrino singlets. In this framework, where lepton-number breaking occurs at a scale much higher than the electroweak scale, deviations from family-lepton-number conservation are calculable and finite, and lepton mixing stems exclusively from M_R. We then show that a discrete symmetry exists such that, in the lepton mixing matrix U, maximal atmospheric neutrino mixing together with U_{e3}=0 can be obtained naturally. Alternatively, if one assumes that there are two different scales in M_R and that the lepton number \bar L = L_e - L_\mu - L_\tau is conserved in between them, then maximal solar neutrino mixing follows naturally. If both the discrete symmetry and intermediate \bar L conservation are introduced, bimaximal mixing is achieved.Comment: Latex, 15 pages; two references added and minor changes in the text; final version accepted for publication in JHE

Lepton mixing and the charged-lepton mass ratios

We construct a class of renormalizable models for lepton mixing that generate predictions given in terms of the charged-lepton mass ratios. We show that one of those models leads, when one takes into account the known experimental values, to almost maximal CP-breaking phases and to almost maximal neutrinoless double-beta decay. We study in detail the scalar potential of the models, especially the bounds imposed by unitarity on the values of the quartic couplings.Comment: 39 pages, 9 figures; revised version addressed the criticism of the editor, matches now the published versio

The seesaw mechanism at arbitrary order: disentangling the small scale from the large scale

We develop a recipe which allows one to recursively and uniquely decouple the large scale from the small scale in mass matrices of the seesaw type, up to any order in the inverse of the large scale. Our method allows one to calculate the mass matrix of the light neutrinos with arbitrary precision. The same method can be applied in the case of quark mass matrices in an extension of the Standard Model with vector-like quarks which have mass terms at a scale much higher than the electroweak scale.Comment: 8 pages, Latex, no figures; paragraph added at the end of section 2, one reference adde

Tri-bimaximal lepton mixing from symmetry only

We construct a model for tri-bimaximal lepton mixing which employs only family symmetries and their soft breaking; neither vacuum alignment nor supersymmetry, extra dimensions, or non-renormalizable terms are used in our model. It is an extension of the Standard Model making use of the seesaw mechanism with five right-handed neutrino singlets. The scalar sector comprises four Higgs doublets and one complex gauge singlet. The horizontal symmetry of our model is based on the permutation group S_3 of the lepton families together with the three family lepton numbers--united this constitutes a symmetry group Delta(6\infty^2). The model makes no predictions for the neutrino masses.Comment: 16 pages, no figures; references added, section 3 supplemented by discussion of the group structure of the mode