121 research outputs found
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 symmetry
associated with a suitably defined family number , together with a
\mathbbm{Z}_2 symmetry which does not commute with . is softly
broken by the mass terms of the right-handed neutrino singlets, which are
responsible for the seesaw mechanism; in addition, 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 ; 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
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
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 , respectively. Both
models have a twofold-degenerate neutrino Dirac mass matrix , a Majorana
mass matrix invariant under a -- interchange symmetry and the
predictions of maximal atmospheric neutrino mixing and vanishing mixing angle
. Leptogenesis can naturally be incorporated if eV where the mass of the lightest neutrino and if
the relevant heavy neutrinos are in the range to GeV. The
model is more constrained and leptogenesis requires to be in the
vicinity of 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
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
Cobimaximal lepton mixing, i.e. and in the lepton mixing matrix , arises as a consequence of , where is the permutation matrix that interchanges the
second and third rows of and is a diagonal matrix of phase
factors. We prove that any such may be written in the form ,
where is any predefined unitary matrix satisfying , is an
orthogonal, i.e. real, matrix, and is a diagonal matrix satisfying . Using this theorem, we demonstrate the equivalence of two ways of
constructing models for cobimaximal mixing---one way that uses a standard
symmetry and a different way that uses a symmetry including --
interchange. We also present two simple seesaw models to illustrate this
equivalence; those models have, in addition to the 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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