188 research outputs found
Fermion masses and mixing from an extra gauge symmetry
Assuming that a horizontal abelian (gauge) symmetry is at the origin of
texture zeros in the fermion mass matrices we show how realistic mass patterns
can be generated in the presence of scalar fields whose vacuum expectation
value breaks the extra symmetry. In the simplest scenario with just one
pair of singlet fields and under the assumption of L-R symmetry one obtains
quark mass matrices {\it \`a la Fritzsch}. The symmetry can be made
anomaly free by the Green-Schwarz mechanism in which case the canonical
unification of the gauge couplings emerges as its byproduct. The generation of
neutrino masses requires either two extra heavy scalar (Higgs) fields to
determine the texture structure of the righthanded neutrino mass matrix or the
latter will contain a hierarchy of scales.Comment: 7 pages of LATE
Orbifold-induced term and electroweak symmetry breaking
It is known that a Higgs term can be naturally generated through the
K\"ahler potential in orbifold string models in which one of the three
compactified complex planes has order two. In this class of models explicit
expressions for both the parameter and the soft SUSY-breaking parameters
can be obtained under the assumption that the goldstino is an arbitrary linear
combination of the fermionic partners of the dilaton and all the moduli
. We apply this picture to the MSSM and explore the consistency of the
obtained boundary conditions with radiative gauge symmetry breaking. We find
that consistency with the measured value of the top-quark mass can only be
achieved if the goldstino has a negligible dilatino component and relevant
components along the moduli associated to the order-two complex
plane.Comment: 9 pages, LaTeX fil
Strings at the Intermediate Scale, or is the Fermi Scale Dual to the Planck Scale?
We show that if the string scale is identifed with the intermediate scale,
GeV, then the notorious hierarchy,
, can be explained using only as small input parameters, where is the
compactification scale. This is possible for weakly-coupled Type-I open-string
vacua if the observed world is assumed to live in an N=1 supersymmetric 3-brane
sector coupled to a separate, hidden, 3-brane world which breaks supersymmetry,
because for such a model . We
discuss some of the phenomenological issues presented by such an
intermediate-scale string, showing that its benefits include:
(i) the possibility of logarithmic gauge-coupling unification of the SM
couplings at ;
(ii) a natural axionic solution to the strong-CP problem with a
phenomenologically-acceptable Peccei-Quinn scale;
(iii) experimentally-interesting neutrino masses, and more.Comment: 16 pages, LaTeX, no figure
Gauging Away the Strong CP Problem
We propose a new solution to the strong-CP problem. It involves the existence
of an unbroken gauged symmetry whose gauge boson gets a Stuckelberg
mass term by combining with a pseudoscalar field . The latter has
axion-like couplings to so that the theta parameter may
be gauged away by a gauge transformation. This system leads to mixed
gauge anomalies and we argue that they are cancelled by the addition of an
appropriate Wess-Zumino term, so that no SM fermions need to be charged under
. We discuss scenarios in which the above set of fields and couplings
appear. The mechanism is quite generic, but a natural possibility is that the
the symmetry arises from bulk gauge bosons in theories with extra
dimensions or string models. We show that in certain D-brane Type-II string
models (with antisymmetric tensor field strength fluxes) higher dimensional
Chern-Simons couplings give rise to the required D=4 Wess-Zumino terms upon
compactification. In one of the possible string realizations of the mechanism
the gauge boson comes from the Kaluza-Klein reduction of the
eleven-dimensional metric in M-theory.Comment: 21 pages, latex, one eps figure; v2 improved discussio
Baryogenesis, Dark Matter and the Pentagon
We present a new mechanism for baryogenesis, which links the baryon asymmetry
of the universe to the dark matter density. The mechanism arises naturally in
the Pentagon model of TeV scale physics. In that context, it forces a
re-evaluation of some of the assumptions of the model, and we detail the
changes that are required in order to fit observations.Comment: JHEP3 LaTeX, 15 pages. New version corrects errors in the electroweak
baryon violating and matter radiation temperatures, which were pointed out by
the referee. Substantial quantitative but no qualitative change to our
conclusion
Building GUTs from strings
We study in detail the structure of Grand Unified Theories derived as the
low-energy limit of orbifold four-dimensional strings. To this aim, new
techniques for building level-two symmetric orbifold theories are presented.
New classes of GUTs in the context of symmetric orbifolds are then constructed.
The method of permutation modding is further explored and SO(10) GUTs with both
or -plets are obtained. SU(5) models are also found through this
method. It is shown that, in the context of symmetric orbifold GUTs,
only a single GUT-Higgs, either a or a , can be present and it always
resides in an order-two untwisted sector. Very restrictive results also hold in
the case of . General properties and selection rules for string GUTs are
described. Some of these selection rules forbid the presence of some particular
GUT-Higgs couplings which are sometimes used in SUSY-GUT model building. Some
semi-realistic string GUT examples are presented and their properties briefly
discussed.Comment: 40 pages, no figures, Late
String GUTs
Standard SUSY-GUTs such as those based on or lead to
predictions for the values of and in amazing
agreement with experiment. In this article we investigate how these models may
be obtained from string theory, thus bringing them into the only known
consistent framework for quantum gravity. String models with matter in standard
GUT representations require the realization of affine Lie algebras at higher
levels. We start by describing some methods to build level orbifold
string models with gauge groups or . We present several
examples and identify generic features of the type of models constructed.
Chiral fields appropriate to break the symmetry down to the standard model
generically appear in the massless spectrum. However, unlike in standard
SUSY-GUTs, they often behave as string moduli, i.e., they do not have
self-couplings. We also discuss briefly the doublet-triplet Higgs splitting. We
find that, in some models, built-in sliding-singlet type of couplings exist.Comment: 40 pages, Latex. Two references modified, few other unimportant
change
Dynamical supersymmetry breaking from unoriented D-brane instantons
We study the non-perturbative dynamics of an unoriented Z_5-quiver theory of
GUT kind with gauge group U(5) and chiral matter. At strong coupling the
non-perturbative dynamics is described in terms of set of baryon/meson
variables satisfying a quantum deformed constraint. We compute the effective
superpotential of the theory and show that it admits a line of supersymmetric
vacua and a phase where supersymmetry is dynamically broken via gaugino
condensation.Comment: 24 pages, 1 figur
Instanton effects in N=1 brane models and the Kahler metric of twisted matter
We consider locally consistent systems of magnetized D9 branes on an
orbifolded six-torus which support N=1 gauge theories. In such realizations,
the matter multiplets arise from "twisted" strings connecting different stacks
of branes. The introduction of Euclidean 5 branes (E5) wrapped on the
six-dimensional compact space leads to instanton effects. For instance, if the
system is engineered so as to yield SQCD, a single E5 brane may account for the
ADS/TVY superpotential. We discuss the subtle interplay that exists between the
annuli diagrams with an E5 boundary and the holomorphicity properties of the
effective low-energy action of the N=1 theory. The consistency of this picture
allows to obtain information on the Kahler metric of the chiral matter
multiplets arising from twisted strings.Comment: 33 pages, 4 figures. V2: improved discussion, clarifyng comments and
references added. Version to be published in JHE
Heterotic/Heterotic Duality in D=6,4
We consider heterotic compactifications on and . The idea of heterotic/heterotic duality in has difficulties for
generic compactifications since for large dilaton values some gauge groups
acquire negative kinetic terms. Recently Duff, Minasian and Witten (DMW)
suggested a solution to this problem which only works if the compactification
is performed assuming the presence of symmetric gauge embeddings on both
's. We consider an alternative in which asymmetric embeddings are possible
and the wrong sign of kinetic terms for large dilaton value is a signal of
spontaneous symmetry breaking. Upon further toroidal compactification to ,
we find that the duals in the DMW case correspond to models in which the
-function of the different group factors verify ,
whereas the asymmetric solutions that we propose have . We
check the consistency of these dualities by studying the different large
limits of the gauge kinetic function. Dual , models can also be
obtained by the operation of appropriate freely acting twists, as shown in
specific examples.Comment: 15 pages, late
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