Gauge boson families in grand unified theories of fermion masses: E_6^4 x S_4

In third quantization the origin of fermion families is easy to understand: the electron field, the muon field and the tau field are identical fields in precisely the same sense as three electrons are identical and undistinguishable particles of a theory of second quantization. In both cases, the permutation of these fields or particles leaves the lagrangian invariant. One can also extend the concept of family to gauge bosons. This can be obtained through the semidirect product of the gauge group with the group of permutations of n objects. In this paper we have studied the group E_6^4 x S_4. We explain why we have chosen E_6 as fundamental gauge group factor and why we start with a model with four gauge boson/fermion families to accommodate and to fit the standard model with only three fermion families. We suggest a possible symmetry breaking pattern of E_6^4 x S_4 that could explain quark, lepton and neutrino masses and mixings.Comment: 21 pages, no figur

S3 x Z2 model for neutrino mass matrices

We propose a model for lepton mass matrices based on the seesaw mechanism, a complex scalar gauge singlet and a horizontal symmetry S_3 \times \mathbbm{Z}_2. In a suitable weak basis, the charged-lepton mass matrix and the neutrino Dirac mass matrix are diagonal, but the vacuum expectation value of the scalar gauge singlet renders the Majorana mass matrix of the right-handed neutrinos non-diagonal, thereby generating lepton mixing. When the symmetry $S_3$ is not broken in the scalar potential, the effective light-neutrino Majorana mass matrix enjoys $\mu$--$\tau$ interchange symmetry, thus predicting maximal atmospheric neutrino mixing together with $U_{e3} = 0$. A partial and less predictive form of $\mu$--$\tau$ interchange symmetry is obtained when the symmetry $S_3$ is softly broken in the scalar potential. Enlarging the symmetry group S_3 \times \mathbbm{Z}_2 by an additional discrete electron-number symmetry \mathbbm{Z}_2^{(e)}, a more predicitive model is obtained, which is in practice indistinguishable from a previous one based on the group $D_4$.Comment: 13 pages, 3 figures, final version for publication in JHE

Renormalization of the neutrino mass operators in the multi-Higgs-doublet Standard Model

We derive the renormalization group equations (RGE) for the flavour coupling matrices of the effective dimension-five operators which yield Majorana neutrino masses in the multi-Higgs-doublet Standard Model; in particular, we consider the case where two different scalar doublets occur in those operators. We also write down the RGE for the scalar-potential quartic couplings and for the Yukawa couplings of that model, in the absence of quarks. As an application of the RGE, we consider two models which, based on a mu-tau interchange symmetry, predict maximal atmospheric neutrino mixing, together with U_{e3} = 0, at the seesaw scale. We estimate the change of those predictions due to the evolution of the coupling matrices of the effective mass operators from the seesaw scale down to the electroweak scale. We derive an upper bound on that change, thereby finding that the radiative corrections to those predictions are in general negligible.Comment: 16 pages, LaTe

A three-parameter model for the neutrino mass matrix

Using the type-II seesaw mechanism with three Higgs doublets phi_alpha (alpha = e, mu, tau) and four Higgs triplets, we build a model for lepton mixing based on a 384-element horizontal symmetry group, generated by the permutation group S_3 and by six Z_2 transformations. The charged-lepton mass matrix is diagonal; the symmetries of the model would require all the three masses m_alpha to be equal, but different vacuum expectation values of the phi_alpha allow the m_alpha to split. The number of parameters in the Majorana neutrino mass matrix m_nu depends on two options: full breaking of the permutation group S_3, or leaving a mu--tau interchange symmetry intact; and hard or spontaneous violation of CP. We discuss in detail the case with the minimal number of three parameters, wherein m_nu is real, symmetric under mu--tau interchange, and has equal diagonal elements. In that case, CP is conserved in lepton mixing, atmospheric neutrino mixing is maximal, and theta_{13} = 0; moreover, the type of neutrino mass spectrum and the absolute neutrino mass scale are sensitive functions of the solar mixing angle.Comment: 16 pages, one eps figure; some clarifications added, contains new section 5, version accepted for publication in J. Phys.

Lepton mixing angle θ13=0\theta_{13} = 0 with a horizontal symmetry D4D_4

We discuss a model for the lepton sector based on the seesaw mechanism and on a $D_4$ family symmetry. The model predicts the mixing angle $\theta_{13}$ to vanish. The solar mixing angle $\theta_{12}$ is free--it will in general be large if one does not invoke finetuning. The model has an enlarged scalar sector with three Higgs doublets, together with two real scalar gauge singlets $\chi_i$ ($i = 1, 2$) which have vacuum expectation values _0$at the seesaw scale. The atmospheric mixing angle$\theta_{23}$is given by$\tan \theta_{23} = _0$/_0$, and it is maximal if the Lagrangian is $D_4$-invariant; but $D_4$ may be broken softly, by a term of dimension two in the scalar potential, and then < \chi_2_0 becomes different from < \chi_1_0. Thus, the strength of the soft $D_4$ breaking controls the deviation of $\theta_{23}$ from $\pi / 4$. The model predicts a normal neutrino mass spectrum ($m_3 > m_2 > m_1$) and allows successful leptogenesis if $m_1 \sim 4 \times 10^{-3} \mathrm{eV}$; these properties of the model are independent of the presence and strength of the soft $D_4$ breaking.Comment: 13 pages, one figur

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

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