3,971 research outputs found
Instanton induced charged fermion and neutrino masses in a minimal Standard Model scenario from intersecting D-branes
String instanton Yukawa corrections from Euclidean D-branes are investigated
in an effective Standard Model theory obtained from the minimal U(3)xU(2)xU(1)
D-brane configuration. In the case of the minimal chiral and Higgs spectrum, it
is found that superpotential contributions are induced by string instantons for
the perturbatively forbidden entries of the up and down quark mass matrices.
Analogous non-perturbative effects generate heavy Majorana neutrino masses and
a Dirac neutrino texture with factorizable Yukawa couplings. For this latter
case, a specific example is worked out where it is shown how this texture can
reconcile the neutrino data.Comment: 17 pages, 3 figure
Electroweak Breaking in Supersymmetric Models
We discuss the mechanism for electroweak symmetry breaking in supersymmetric
versions of the standard model. After briefly reviewing the possible sources of
supersymmetry breaking, we show how the required pattern of symmetry breaking
can automatically result from the structure of quantum corrections in the
theory. We demonstrate that this radiative breaking mechanism works well for a
heavy top quark and can be combined in unified versions of the theory with
excellent predictions for the running couplings of the model. (To be published
in ``Perspectives in Higgs Physics'', G. Kane editor.)Comment: 47 page
A D-Brane Alternative to the MSSM
The success of SU(5)-like gauge coupling unification boundary conditions
has biased most attempts to embed the SM interactions
into a unified structure. After discussing the limitations of the orthodox
approach, we propose an alternative that appears to be quite naturally implied
by recent developments based on D-brane physics. In this new alternative: 1)
The gauge group, above a scale of order 1 TeV, is the minimal left-right
symmetric extension of the
SM; 2) Quarks, leptons and Higgs fields come in three generations; 3) Couplings
unify at an intermediate string scale GeV with boundary
conditions . This corresponds to the natural
embedding of gauge interactions into D-branes and is different from the
standard SO(10) embedding which corresponds to . Unification only
works in the case of three generations;
4) Proton stability is automatic due to the presence of discrete
R-parity and lepton parities. A specific Type IIB string orientifold model with
the above characteristics is constructed. The existence of three generations is
directly related to the existence of three complex extra dimensions.
In this model the string scale can be identified with the intermediate scale
and SUSY is broken also at that scale due to the presence of anti-branes in the
vacuum.
We discuss a number of phenomenological issues in this model including Yukawa
couplings and a built-in axion solution to the strong-CP problem. The present
framework could be tested by future accelerators by finding the left-right
symmetric extension of the SM at a scale of order 1 TeV.Comment: 50 pages, 7 figures. References adde
Standard-like Models with Broken Supersymmetry from Type I String Vacua
We construct D=4 Type I vacua with massless content remarkably close to that
of the standard model of particle physics. They are tachyon-free
non-supersymmetric models which are obtained starting with a standard D=4, N=1
compact Type IIB orientifold and adding the same number of Dp-branes and
anti-Dp-branes distributed at different points of the underlying orbifold.
Supersymmetry-breaking is felt by the observable world either directly, by
gravity mediation or gauge mediation, depending on the brane configuration. We
construct several simple three generation examples with the gauge group of the
standard model or its left-right symmetric extensions. The models contain a
number of U(1) gauge groups whose anomalies are cancelled by a generalized
Green-Schwarz mechanism. These U(1)'s are broken but may survive as global
symmetries providing for a flavour structure to the models. The value of the
string scale may be lowered down to the intermediate scale (as required in the
gravity mediation case) or down to 1-100 TeV for the non-SUSY models. Thus the
present models are the first semirealistic string vacua realizing the
possibility of a low string scale. The unbalanced force between the pairs of
Dp- and anti-Dp-branes provides for an effect which tends to compactify some of
the extra dimensions but no others. This could provide a new mechanism for
radius stabilization.Comment: 58 pages, latex (typos corrected and references added
Flux-induced Soft Terms on Type IIB/F-theory Matter Curves and Hypercharge Dependent Scalar Masses
Closed string fluxes induce generically SUSY-breaking soft terms on
supersymmetric type IIB orientifold compactifications with D3/D7 branes. This
was studied in the past by inserting those fluxes on the DBI+CS actions for
adjoint D3/D7 fields, where D7-branes had no magnetic fluxes. In the present
work we generalise those computations to the phenomenologically more relevant
case of chiral bi-fundamental fields laying at 7-brane intersections and
F-theory local matter curves. We also include the effect of 7-brane magnetic
flux as well as more general closed string backgrounds, including the effect of
distant (anti-)D3-branes. We discuss several applications of our results. We
find that squark/slepton masses become in general flux-dependent in F-theory
GUT's. Hypercharge-dependent non-universal scalar masses with a characteristic
sfermion hierarchy m_E^2 < m_L^2 < m_Q^2 < m_D^2 < m_U^2 are obtained. There
are also flavor-violating soft terms both for matter fields living at
intersecting 7-branes or on D3-branes at singularities. They point at a very
heavy sfermion spectrum to avoid FCNC constraints. We also discuss the possible
microscopic description of the fine-tuning of the EW Higgs boson in
compactifications with a MSSM spectrum.Comment: 67 pages, 2 figures, 2 table
Providing structural modules with self-integrity monitoring
With the advent of complex space structures (i.e., U.S. Space Station), the need for methods for remotely detecting structural damage will become greater. Some of these structures will have hundreds of individual structural elements (i.e., strut members). Should some of them become damaged, it could be virtually impossible to detect it using visual or similar inspection techniques. The damage of only a few individual members may or may not be a serious problem. However, should a significant number of the members be damaged, a significant problem could be created. The implementation of an appropriate remote damage detection scheme would greatly reduce the likelihood of a serious problem related to structural damage ever occurring. This report presents the results of the research conducted on remote structural damage detection approaches and the related mathematical algorithms. The research was conducted for the Small Business Innovation and Research (SBIR) Phase 2 National Aeronautics and Space Administration (NASA) Contract NAS7-961
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