7 research outputs found
Large theta_13 from a model with broken L_e-L_mu-L_tau symmetry
Recent data in the neutrino sector point towards a relatively large value of
the reactor angle, incompatible with a vanishing theta_ 13 at about 3 sigma. In
order to explain such a result, we propose a SUSY model based on the broken
L_e-L_mu-L_tau symmetry, where large deviations from the symmetric limit
theta_12 = pi/4, tan(theta_23) \sim O(1) and theta_13 = 0 mainly come from the
charged lepton sector. We show that a description of all neutrino data is
possible if the charged lepton mass matrix has a special pattern of complex
matrix elements.Comment: 9 pages, 2 figures. v2: comments and references added, as published
in JHE
Non-Abelian family symmetries in Pati-Salam unification
We present a framework of underlying SU(3) x SU(3) family symmetries
consistent with Pati-Salam unification and discuss advantages that can justify
introducing multiple non-Abelian factors. Advantages include improved vacuum
alignment and increased predictivity. We explore in this framework deviations
from tri-bi-maximal neutrinos, such as relatively large theta13.Comment: 11 pages, final version for publication in JHE
Leptons in Holographic Composite Higgs Models with Non-Abelian Discrete Symmetries
We study leptons in holographic composite Higgs models, namely in models
possibly admitting a weakly coupled description in terms of five-dimensional
(5D) theories. We introduce two scenarios leading to Majorana or Dirac
neutrinos, based on the non-abelian discrete group which is
responsible for nearly tri-bimaximal lepton mixing. The smallness of neutrino
masses is naturally explained and normal/inverted mass ordering can be
accommodated. We analyze two specific 5D gauge-Higgs unification models in
warped space as concrete examples of our framework. Both models pass the
current bounds on Lepton Flavour Violation (LFV) processes. We pay special
attention to the effect of so called boundary kinetic terms that are the
dominant source of LFV. The model with Majorana neutrinos is compatible with a
Kaluza-Klein vector mass scale TeV, which is roughly the
lowest scale allowed by electroweak considerations. The model with Dirac
neutrinos, although not considerably constrained by LFV processes and data on
lepton mixing, suffers from a too large deviation of the neutrino coupling to
the boson from its Standard Model value, pushing TeV.Comment: 37 pages, 4 figures; v2: Note added in light of recent T2K and MINOS
results, figures updated with new limit from MEG, references added, various
minor improvements, matches JHEP published versio
Discrete Flavour Groups, \theta_13 and Lepton Flavour Violation
Discrete flavour groups have been studied in connection with special patterns
of neutrino mixing suggested by the data, such as Tri-Bimaximal mixing (groups
A4, S4...) or Bi-Maximal mixing (group S4...) etc. We review the predictions
for sin(\theta_13) in a number of these models and confront them with the
experimental measurements. We compare the performances of the different classes
of models in this respect. We then consider, in a supersymmetric framework, the
important implications of these flavour symmetries on lepton flavour violating
processes, like \mu -> e gamma and similar processes. We discuss how the
existing limits constrain these models, once their parameters are adjusted so
as to optimize the agreement with the measured values of the mixing angles. In
the simplified CMSSM context, adopted here just for indicative purposes, the
small tan(beta) range and heavy SUSY mass scales are favoured by lepton flavour
violating processes, which makes it even more difficult to reproduce the
reported muon g-2 discrepancy.Comment: 45 pages, 16 figures, 3 tables; V3 submitted to add an acknowledgment
to a Networ
Natural Vacuum Alignment from Group Theory: The Minimal Case
Discrete flavour symmetries have been proven successful in explaining the
leptonic flavour structure. To account for the observed mixing pattern, the
flavour symmetry has to be broken to different subgroups in the charged and
neutral lepton sector. However, cross-couplings via non-trivial contractions in
the scalar potential force the group to break to the same subgroup. We present
a solution to this problem by extending the flavour group in such a way that it
preserves the flavour structure, but leads to an 'accidental' symmetry in the
flavon potential. We have searched for symmetry groups up to order 1000, which
forbid all dangerous cross-couplings and extend one of the interesting groups
A4, T7, S4, T' or \Delta(27). We have found a number of candidate groups and
present a model based on one of the smallest extension of A4, namely Q8 \rtimes
A4. We show that the most general non-supersymmetric potential allows for the
correct vacuum alignment. We investigate the effects of higher dimensional
operators on the vacuum configuration and mixing angles, and give a
see-saw-like UV completion. Finally, we discuss the supersymmetrization of the
model. Additionally, we release the Mathematica package "Discrete" providing
various useful tools for model building such as easily calculating invariants
of discrete groups and flavon potentials.Comment: 33 pages, 7 figures; references added, minor changes, matches version
published in JHE
A4 Flavor Models in Split Seesaw Mechanism
A seesaw mechanism in an extra-dimension, known as the split seesaw
mechanism, provides a natural way to realize a splitting mass spectrum of
right-handed neutrinos. It leads to one keV sterile neutrino as a dark matter
candidate and two heavy right-handed neutrinos being responsible for
leptogenesis to explain the observed baryon asymmetry of the Universe. We study
models based on flavor symmetry in the context of the split seesaw
mechanism. It is pointed out that most of known flavor models with three
right-handed neutrinos being triplet suffer from a degeneracy problem for
the bulk mass terms, which disturbs the split mechanism for right-handed
neutrino mass spectrum. Then we construct a new flavor model to work in
the split seesaw mechanism. In the model, the experimentally observed neutrino
masses and mixing angles can be realized from both type I+II seesaw
contributions. The model predicts the symmetry in the neutrino mass
matrix at the leading order, resulting in the vanishing and
maximal . The flavor symmetry is broken via the flavon
vacuum alignment which can be obtained from the orbifold compactification. The
model can be consistent with all data of neutrino oscillation experiments,
cosmological discussions of dark matter abundance, leptogenesis, and recent
astrophysical data.Comment: 21 pages, 1 figure, version to appear in JHE