121 research outputs found

    Maximal atmospheric neutrino mixing in an SU(5) model

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    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)FU(1)_F symmetry associated with a suitably defined family number FF, together with a \mathbbm{Z}_2 symmetry which does not commute with FF. U(1)FU(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)FU(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 1016GeV<mU<1019GeV10^{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

    Tri-bimaximal lepton mixing from symmetry only

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    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

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    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

    A model for trimaximal lepton mixing

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    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

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    We discuss two extensions of the Standard Model based on the seesaw mechanism and on non-abelian family symmetry groups O(2) and D4D_4, respectively. Both models have a twofold-degenerate neutrino Dirac mass matrix MDM_D, a Majorana mass matrix invariant under a μ\mu--τ\tau interchange symmetry and the predictions of maximal atmospheric neutrino mixing and vanishing mixing angle θ13\theta_{13}. Leptogenesis can naturally be incorporated if 10−3≲m1≲10−210^{-3} \lesssim m_1 \lesssim 10^{-2} eV where m1m_1 the mass of the lightest neutrino and if the relevant heavy neutrinos are in the range 101110^{11} to 101210^{12} GeV. The D4D_4 model is more constrained and leptogenesis requires m1m_1 to be in the vicinity of 4×10−34 \times 10^{-3} eV.Comment: 8 pages, one figure, talk presented at NOON2004, February 11-15, 2004, Tokyo, Japa

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

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    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

    Cobimaximal lepton mixing from soft symmetry breaking

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    Cobimaximal lepton mixing, i.e. θ23=45∘\theta_{23} = 45^\circ and δ=±90∘\delta = \pm 90^\circ in the lepton mixing matrix VV, arises as a consequence of SV=V∗PS V = V^\ast \mathcal{P}, where SS is the permutation matrix that interchanges the second and third rows of VV and P\mathcal{P} is a diagonal matrix of phase factors. We prove that any such VV may be written in the form V=URPV = U R P, where UU is any predefined unitary matrix satisfying SU=U∗S U = U^\ast, RR is an orthogonal, i.e. real, matrix, and PP is a diagonal matrix satisfying P2=PP^2 = \mathcal{P}. Using this theorem, we demonstrate the equivalence of two ways of constructing models for cobimaximal mixing---one way that uses a standard CPCP symmetry and a different way that uses a CPCP symmetry including μ\mu--τ\tau interchange. We also present two simple seesaw models to illustrate this equivalence; those models have, in addition to the CPCP symmetry, flavour symmetries broken softly by the Majorana mass terms of the right-handed neutrino singlets. Since each of the two models needs four scalar doublets, we investigate how to accommodate the Standard Model Higgs particle in them.Comment: 16 pages, no figure
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