2,405 research outputs found
Minimal string-scale unification of gauge couplings
We look for the minimal particle content which is necessary to add to the
standard model in order to have a complete unification of gauge couplings and
gravity at the weakly coupled heterotic string scale. Using the current
precision electroweak data, we find that the presence of a vector-like fermion
at an intermediate scale and a non-standard hypercharge normalization are in
general sufficient to achieve this goal at two-loop level. If one requires the
extra matter scale to be below the TeV scale, then it is found that the
addition of three vector-like fermion doublets with a mass around 700 GeV
yields a perfect string-scale unification, provided that the affine levels are
kY=13/3, k2=1 and k3=2, as in the SU(5) X SU(5) string-GUT. Furthermore, if
supersymmetry is broken at the unification scale, the Higgs mass is predicted
in the range 125 GeV - 170 GeV, depending on the precise values of the top
quark mass and tan(beta) parameter.Comment: 12 pages, 2 figures, references updated and comments added, final
version to appear in Phys. Lett.
The flavor of product-group GUTs
The doublet-triplet splitting problem can be simply solved in product-group
GUT models, using a global symmetry that distinguishes the doublets from the
triplets. Apart from giving the required mass hierarchy, this ``triplet
symmetry'' can also forbid some of the triplet couplings to matter. We point
out that, since this symmetry is typically generation-dependent, it gives rise
to non-trivial flavor structure. Furthermore, because flavor symmetries cannot
be exact, the triplet-matter couplings are not forbidden then but only
suppressed. We construct models in which the triplet symmetry gives acceptable
proton decay rate and fermion masses. In some of the models, the prediction m_b
~ m_\tau is retained, while the similar relation for the first generation is
corrected. Finally, all this can be accomplished with triplets somewhat below
the GUT scale, supplying the right correction for the standard model gauge
couplings to unify precisely.Comment: 10 page
Texture Zeros and Weak Basis Transformations
We investigate the physical meaning of some of the "texture zeros" which
appear in most of the Ansatze on quark masses and mixings. It is shown that
starting from arbitrary quark mass matrices and making a suitable weak basis
transformation one can obtain some of these sets of zeros which therefore have
no physical content. We then analyse the physical implications of a
four-texture zero Ansatz which is in agreement with all present experimental
data.Comment: 11 pages, typeset using revte
Operator Analysis for Proton Decay in SUSY SO(10) GUT Models
Non-renormalizable operators both account for the failure of down quark and
charged lepton Yukawa couplings to unify and reduce the proton decay rate via
dimension-five operators in minimal SUSY SU(5) GUT. We extend the analysis to
SUSY SO(10) GUT models.Comment: Higgs sector clarified, two Refs adde
Proton Stability In Supersymmetric SU(5)
Within supersymmetric SU(5) GUT we suggest mechanisms for suppression of
baryon number violating dimension five and six operators. The mechanism is
based on the idea of split multiplets (i.e. quarks and leptons are not coming
from a single GUT state) which is realized by an extension with additional
vector-like matter. The construction naturally avoids wrong asymptotic relation
. Thus, the long standing problems of the minimal SUSY
SU(5) GUT can be resolved.
In a particular example of flavor structure and with additional {\cal
U}(1)\tm {\cal Z}_{3N} symmetry we demonstrate how the split multiplet
mechanism works out. Namely, the considered model is compatible with successful
gauge coupling unification and realistic fermion mass pattern. The nucleon
decay rates are relatively suppressed and can be well compatible with current
experimental bounds.Comment: Discussions and some clarifications adde
Minimal anomaly-free chiral fermion sets and gauge coupling unification
We look for minimal chiral sets of fermions beyond the standard model that
are anomaly free and, simultaneously, vectorlike particles with respect to
colour SU(3) and electromagnetic U(1). We then study whether the addition of
such particles to the standard model particle content allows for the
unification of gauge couplings at a high energy scale, above GeV so as to be safely consistent with proton decay bounds. The
possibility to have unification at the string scale is also considered.
Inspired in grand unified theories, we also search for minimal chiral fermion
sets that belong to SU(5) multiplets. Restricting to representations up to
dimension 50, we show that some of these sets can lead to gauge unification at
the GUT and/or string scales.Comment: 13 pages, 5 figures, 8 tables; Comments and references added, final
version to appear in Phys. Rev.
Spontaneous symmetry breaking in the -symmetric scalar sector
We present a detailed study of the vacua of the -symmetric
three-Higgs-doublet potential, specifying the region of parameters where these
minimisation solutions occur. We work with a CP conserving scalar potential and
analyse the possible real and complex vacua with emphasis on the cases in which
the CP symmetry can be spontaneously broken. Results are presented both in the
reducible-representation framework of Derman, and in the
irreducible-representation framework. Mappings between these are given. Some of
these implementations can in principle accommodate dark matter and for that
purpose it is important to identify the residual symmetries of the potential
after spontaneous symmetry breakdown. We are also concerned with constraints
from vacuum stability.Comment: 37 pages. v2: Minor changes in the references, matches published
version. v3: Table 6 corrected: two additional cases conserve CP. Related
discussion adapted. Version consistent with JHEP Erratu
Reconstruction of Quark Mass Matrices with Weak Basis Texture Zeroes from Experimental Input
All quark mass matrices with texture zeroes obtained through weak basis
transformations are confronted with the experimental data. The reconstruction
of the quark mass matrices M_u and M_d at the electroweak scale is performed in
a weak basis where the matrices are Hermitian and have a maximum of three
vanishing elements. The same procedure is also accomplished for the Yukawa
coupling matrices at the grand unification scale in the context of the Standard
Model and its minimal supersymmetric extension as well as of the two Higgs
doublet model. The analysis of all viable power structures on the quark Yukawa
coupling matrices that could naturally appear from a Froggatt-Nielsen mechanism
is also presented.Comment: RevTeX4, 3 tables, 21 pages; misprints corrected and one reference
adde
Spontaneous symmetry breaking in three-Higgs-doublet -symmetric models
The talk summarises work done by the authors consisting of a detailed study
of the possible vacua in models with three Higgs doublets with symmetry
and without explicit CP violation. Different vacua require special regions of
the parameter space which were analysed in our work. We establish the
possibility of spontaneous CP violation in this framework and we also show
which complex vacua conserve CP. In our work we discussed constraints from
vacuum stability. The results presented here are relevant for model building.Comment: 11 pages, no figures. Prepared for the proceedings of DISCRETE2016:
the Fifth Symposium on Prospects in the Physics of Discrete Symmetries, 28
November-3 December 2016, University of Warsaw, Poland, to appear in the
Journal of Physics: Conference Series (JPCS
Proton Decay in a Consistent Supersymmetric SU(5) GUT Model
It is widely believed that minimal supersymmetric SU(5) GUTs have been excluded by the SuperKamiokande bound for the proton decay rate. In the minimal model, however, the theoretical prediction assumes unification of Yukawa couplings, Y_d=Y_e, which is known to be badly violated. We analyze the implications of this fact for the proton decay rate. In a consistent SU(5) model with higher dimensional operators, where SU(5) relations among Yukawa couplings hold, the proton decay rate can be several orders of magnitude smaller than the present experimental bound
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