207 research outputs found
Discrete symmetries for electroweak natural type-I seesaw mechanism
The naturalness of electroweak scale in the models of type-I seesaw mechanism
with Yukawa couplings requires TeV scale masses for the fermion
singlets. In this case, the tiny neutrino masses have to arise from the
cancellations within the seesaw formula which are arranged by fine-tuned
correlations between the Yukawa couplings and the masses of fermion singlets.
We motivate such correlations through the framework of discrete symmetries. In
the case of three Majorana fermion singlets, it is shown that the exact
cancellation arranged by the discrete symmetries in seesaw formula necessarily
leads to two mass degenerate fermion singlets. The remaining fermion singlet
decouples completely from the standard model. We provide two candidate models
based on the groups and and discuss the generic
perturbations to this approach which can lead to the viable neutrino masses.Comment: 26 pages, 4 figures; references added, matches published versio
Proton decay in flux compactifications
We study proton decay in a six-dimensional orbifold GUT model with gauge
group . Magnetic flux in the compact dimensions
determines the multiplicity of quark-lepton generations, and it also breaks
supersymmetry by giving universal GUT scale masses to scalar quarks and
leptons. The model can successfully account for quark and lepton masses and
mixings. Our analysis of proton decay leads to the conclusion that the proton
lifetime must be close to the current experimental lower bound. Moreover, we
find that the branching ratios for the decay channels
and are of similar size, in fact the latter one can
even be dominant. This is due to flavour non-diagonal couplings of heavy vector
bosons together with large off-diagonal Higgs couplings, which appears to be a
generic feature of flux compactifications.Comment: 26 pages, 3 figures, 2 table
Flavour physics without flavour symmetries
We quantitatively analyze a quark-lepton flavour model derived from a
six-dimensional supersymmetric theory with gauge symmetry,
compactified on an orbifold with magnetic flux. Two bulk -plets
charged under the provide the three quark-lepton generations whereas two
uncharged -plets yield two Higgs doublets. At the orbifold fixed
points mass matrices are generated with rank one or two. Moreover, the zero
modes mix with heavy vectorlike split multiplets. The model possesses no
flavour symmetries. Nevertheless, there exist a number of relations between
Yukawa couplings, remnants of the underlying GUT symmetry and the wave function
profiles of the zero modes, which lead to a prediction of the light neutrino
mass scale, eV and heavy Majorana neutrino masses in
the range from GeV to GeV. The model successfully includes
thermal leptogenesis.Comment: Minor additions; Published versio
Interpreting 750 GeV diphoton excess in SU(5) grand unified theory
The ATLAS and CMS experiments at the LHC have found significant excess in the
diphoton invariant mass distribution near 750 GeV. We interpret this excess in
a predictive nonsupersymmetric SU(5) grand unified framework with a singlet
scalar and light adjoint fermions. The 750 GeV resonance is identified as a
gauge singlet scalar. Both its production and decays are induced by 24
dimensional adjoint fermions predicted within SU(5). The adjoint fermions are
assumed to be odd under symmetry which forbids their direct coupling to
the standard model fermions. We show that the observed diphoton excess can be
explained with sub-TeV adjoint fermions and with perturbative Yukawa coupling.
A narrow width scenario is more preferred while a simultaneous explanation of
observed cross section and large total decay width requires some of the adjoint
fermions lighter than 375 GeV. The model also provides a singlet fermion as a
candidate of cold dark matter. The gauge coupling unification is achieved in
the framework by introducing color sextet scalars while being consistent with
the proton decay constraint.Comment: Discussion added, conclusion unchanged; Matches published version in
Physics Letters
Revisiting lepton flavor violation in supersymmetric type II seesaw
In view of the recent measurement of reactor mixing angle and
updated limit on by the MEG experiment, we re-examine
the charged lepton flavor violations in a framework of supersymmetric type II
seesaw mechanism. Supersymmetric type II seesaw predicts strong correlation
between and mainly in terms of
the neutrino mixing angles. We show that such a correlation can be determined
accurately after the measurement of . We compute different factors
which can affect this correlation and show that the mSUGRA-like scenarios, in
which slepton masses are taken to be universal at the high scale, predicts for normal
hierarchical neutrino masses. Any experimental indication of deviation from
this prediction would rule out the minimal models of supersymmetric type II
seesaw. We show that the current MEG limit puts severe constraints on the light
sparticle spectrum in mSUGRA model if the seesaw scale lies within
- GeV. It is shown that these constraints can be relaxed and
relatively light sparticle spectrum can be obtained in a class of models in
which the soft mass of triplet scalar is taken to be non-universal at the high
scale.Comment: Minor changes in text; accepted for publication in Phys. Rev.
Fermion Masses in SO(10) Models
We examine many SO(10) models for their viability or otherwise in explaining
all the fermion masses and mixing angles. This study is carried out for both
supersymmetric and non-supersymmetric models and with minimal ()
and non-minimal () Higgs content. Extensive numerical fits to
fermion masses and mixing are carried out in each case assuming dominance of
type-II or type-I seesaw mechanism. Required scale of the B-L breaking is
identified in each case. In supersymmetric case, several sets of data at the
GUT scale with or without inclusion of finite supersymmetric corrections are
used. All models studied provide quite good fits if the type-I seesaw mechanism
dominates while many fail if the type-II seesaw dominates. This can be traced
to the absence of the - unification at the GUT scale in these models.
The minimal non-supersymmetric model with type-I seesaw dominance gives
excellent fits. In the presence of a and an intermediate scale, the
model can also account for the gauge coupling unification making it potentially
interesting model for the complete unification. Structure of the Yukawa
coupling matrices obtained numerically in this specific case is shown to follow
from a very simple U(1) symmetry and a Froggatt-Nielsen singlet.Comment: 31 pages, 9 Tables, 4 figure
Generalized - symmetry and discrete subgroups of O(3)
The generalized - interchange symmetry in the leptonic mixing
matrix corresponds to the relations: with
. It predicts maximal atmospheric mixing and maximal Dirac CP
violation given . We show that the generalized -
symmetry can arise if the charged lepton and neutrino mass matrices are
invariant under specific residual symmetries contained in the finite discrete
subgroups of . The groups , and are the only such groups
which can entirely fix at the leading order. The neutrinos can be (a)
non-degenerate or (b) partially degenerate depending on the choice of their
residual symmetries. One obtains either vanishing or very large
in case of (a) while only can provide close to its
experimental value in the case (b). We provide an explicit model based on
and discuss a class of perturbations which can generate fully realistic
neutrino masses and mixing maintaining the generalized - symmetry in
. Our approach provides generalization of some of the ideas proposed earlier
in order to obtain the predictions, and .Comment: 18 page
Pseudo-Dirac neutrinos from flavour dependent CP symmetry
Discrete residual symmetries and flavour dependent CP symmetries consistent
with them have been used to constrain neutrino mixing angles and CP violating
phases. We discuss here role of such CP symmetries in obtaining a pseudo-Dirac
neutrino which can provide a pair of neutrinos responsible for the solar
splitting. It is shown that if (a) Majorana neutrino matrix
is invariant under a discrete symmetry generated by ,
(b) CP symmetry transform as , and (c) and
obey consistency conditions , then two
of the neutrino masses are degenerate independent of specific forms of ,
and . Explicit examples of this result are discussed in the context
of groups which can also be used to constrain neutrino mixing
matrix . Degeneracy in two of the masses does not allow complete
determination of but it can also be fixed once the perturbations are
introduced. We consider explicit perturbations which break
symmetries but respect CP. These are shown to remove the degeneracy and provide
a predictive description of neutrino spectrum. In particular, a correlation
is obtained between the
atmospheric mixing angle and the CP violating phase
in terms of a group theoretically determined phase factor . Experimentally
interesting case ,
emerges for groups which predict purely imaginary . We present detailed
predictions of the allowed ranges of neutrino mixing angles, phases and the
lightest neutrino mass for three of the lowest groups with
.Comment: 17 pages, 4 figures; Minor modification, published versio
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