181 research outputs found
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
Ultraviolet Completion of Flavour Models
Effective Flavour Models do not address questions related to the nature of
the fundamental renormalisable theory at high energies. We study the
ultraviolet completion of Flavour Models, which in general has the advantage of
improving the predictivity of the effective models. In order to illustrate the
important features we provide minimal completions for two known A4 models. We
discuss the phenomenological implications of the explicit completions, such as
lepton flavour violating contributions that arise through the exchange of
messenger fields.Comment: 18 pages, 8 figure
Trimaximal neutrino mixing from vacuum alignment in A4 and S4 models
Recent T2K results indicate a sizeable reactor angle theta_13 which would
rule out exact tri-bimaximal lepton mixing. We study the vacuum alignment of
the Altarelli-Feruglio A4 family symmetry model including additional flavons in
the 1' and 1" representations and show that it leads to trimaximal mixing in
which the second column of the lepton mixing matrix consists of the column
vector (1,1,1)^T/sqrt{3}, with a potentially large reactor angle. In order to
limit the reactor angle and control the higher order corrections, we propose a
renormalisable S4 model in which the 1' and 1" flavons of A4 are unified into a
doublet of S4 which is spontaneously broken to A4 by a flavon which enters the
neutrino sector at higher order. We study the vacuum alignment in the S4 model
and show that it predicts accurate trimaximal mixing with approximate
tri-bimaximal mixing, leading to a new mixing sum rule testable in future
neutrino experiments. Both A4 and S4 models preserve form dominance and hence
predict zero leptogenesis, up to renormalisation group corrections.Comment: 24 pages, 2 figures, version to be published in JHE
An SO(10) Grand Unified Theory of Flavor
We present a supersymmetric SO(10) grand unified theory (GUT) of flavor based
on an family symmetry. It makes use of our recent proposal to use SO(10)
with type II seesaw mechanism for neutrino masses combined with a simple ansatz
that the dominant Yukawa matrix (the {\bf 10}-Higgs coupling to matter) has
rank one. In this paper, we show how the rank one model can arise within some
plausible assumptions as an effective field theory from vectorlike {\bf 16}
dimensional matter fields with masses above the GUT scale. In order to obtain
the desired fermion flavor texture we use flavon multiplets which acquire
vevs in the ground state of the theory. By supplementing the theory with
an additional discrete symmetry, we find that the flavon vacuum field
alignments take a discrete set of values provided some of the higher
dimensional couplings are small. Choosing a particular set of these vacuum
alignments appears to lead to an unified understanding of observed quark-lepton
flavor:
(i) the lepton mixing matrix that is dominantly tri-bi-maximal with small
corrections related to quark mixings; (ii) quark lepton mass relations at GUT
scale: and and (iii) the solar to
atmospheric neutrino mass ratio in agreement with observations. The model predicts the neutrino
mixing parameter, ,
which should be observable in planned long baseline experiments.Comment: Final version of the paper as it will appear in JHEP
Bridging flavour violation and leptogenesis in SU(3) family models
We reconsider basic, in the sense of minimal field content, Pati-Salam x
SU(3) family models which make use of the Type I see-saw mechanism to reproduce
the observed mixing and mass spectrum in the neutrino sector. The goal of this
is to achieve the observed baryon asymmetry through the thermal decay of the
lightest right-handed neutrino and at the same time to be consistent with the
expected experimental lepton flavour violation sensitivity. This kind of models
have been previously considered but it was not possible to achieve a
compatibility among all of the ingredients mentioned above. We describe then
how different SU(3) messengers, the heavy fields that decouple and produce the
right form of the Yukawa couplings together with the scalars breaking the SU(3)
symmetry, can lead to different Yukawa couplings. This in turn implies
different consequences for flavour violation couplings and conditions for
realizing the right amount of baryon asymmetry through the decay of the
lightest right-handed neutrino. Also a highlight of the present work is a new
fit of the Yukawa textures traditionally embedded in SU(3) family models.Comment: 26 pages, 5 figures, Some typos correcte
Generalised CP and Family Symmetry in Semi-Direct Models of Leptons
We perform a detailed analysis of family symmetry combined
with a generalised CP symmetry in the lepton sector, breaking to different
remnant symmetries in the neutrino and in the charged lepton
sector, together with different remnant CP symmetries in each sector. We
discuss the resulting mass and mixing predictions for with
and with . All cases correspond to
the preserved symmetry smaller than the full Klein symmetry, as in the
semi-direct approach, leading to predictions which depend on a single
undetermined real parameter, which mainly determines the reactor angle. We
focus on five phenomenologically allowed cases for which we present the
resulting predictions for the PMNS parameters as a function of , as well as
the predictions for neutrinoless double beta decay.Comment: 65 pages, 19 figures, and the predictions for neutrinoless double
beta decay are update
Beyond MFV in family symmetry theories of fermion masses
Minimal Flavour Violation (MFV) postulates that the only source of flavour
changing neutral currents and CP violation, as in the Standard Model, is the
CKM matrix. However it does not address the origin of fermion masses and mixing
and models that do usually have a structure that goes well beyond the MFV
framework. In this paper we compare the MFV predictions with those obtained in
models based on spontaneously broken (horizontal) family symmetries, both
Abelian and non-Abelian. The generic suppression of flavour changing processes
in these models turns out to be weaker than in the MFV hypothesis. Despite
this, in the supersymmetric case, the suppression may still be consistent with
a solution to the hierarchy problem, with masses of superpartners below 1 TeV.
A comparison of FCNC and CP violation in processes involving a variety of
different family quantum numbers should be able to distinguish between various
family symmetry models and models satisfying the MFV hypothesis.Comment: 34 pages, no figure
Discrete family symmetry, Higgs mediators and theta_{13}
We present a new (supersymmetric) framework for obtaining an excellent
description of quark, charged lepton and neutrino masses and mixings from a
Delta(6n^2) family symmetry with multiplet assignments consistent with an
underlying SO(10) Grand Unification. It employs a Higgs mediator sector in
place of the usual Froggatt-Nielsen messengers, with quark and lepton
messengers, and provides significant improvements over existing models of this
type having unsuppressed Yukawa couplings to the third generation and a
simplified vacuum alignment mechanism. The neutrino mass differences are
naturally less hierarchical than those of the quarks and charged leptons.
Similarly the lepton mixing angles are much larger than those in the quark
sector and have an approximate tri-bi-maximal (TB) mixing form for theta_{12}
and theta_{23}. However the mixing angle theta_{13} is naturally much larger
than in pure TB mixing and can be consistent with the value found in recent
experiments. The magnitude of theta_{13} is correlated with a the predicted
deviation of theta_{23} from bi-maximal mixing. The model has light familon
fields that can significantly modify the associated SUSY phenomenology.Comment: v3: accepted in JHE
- …