430 research outputs found
Tachyon-free Non-supersymmetric Type IIB Orientifolds via Brane-Antibrane Systems
We derive the rules to construct type IIB compact orientifolds in six and
four dimensions including D-branes and anti-D-branes. Even though the models
are non-supersymmetric due to the presence of the anti-D-branes, we show that
it is easy to construct large classes of models free of tachyons.
Brane-antibrane annihilation can be prevented for instance by considering
models with branes and antibranes stuck at different fixed points in the
compact space. We construct several anomaly-free and tachyon-free
six-dimensional orientifolds containing D9-branes and anti-D5-branes. This
setup allows to construct four-dimensional chiral models with supersymmetry
unbroken in the bulk and in some D-brane sectors, whereas supersymmetry is
broken (at the string scale) in some `hidden' anti-D-brane sector. We present
several explicit models of this kind. We also comment on the role of the
non-cancelled attractive brane-antibrane forces and the non-vanishing
cosmological constant, as providing interesting dynamics for the geometric
moduli and the dilaton, which may contribute to their stabilization.Comment: 30 pages, Latex, no figures. Minor correction
Tadpole versus anomaly cancellation in D=4,6 compact IIB orientifolds
It is often stated in the literature concerning D=4,6 compact Type IIB
orientifolds that tadpole cancellation conditions i) uniquely fix the gauge
group (up to Wilson lines and/or moving of branes) and ii) are equivalent to
gauge anomaly cancellation. We study the relationship between tadpole and
anomaly cancellation conditions and qualify both statements. In general the
tadpole cancellation conditions imply gauge anomaly cancellation but are
stronger than the latter conditions in D=4, N=1 orientifolds. We also find that
tadpole cancellation conditions in Z_N D=4,6 compact orientifolds do not
completely fix the gauge group and we provide new solutions different from
those previously reported in the literature.Comment: 28 pages, Latex. Minor corrections, updated reference
Type IIB Orientifolds without Untwisted Tadpoles, and non-BPS D-branes
We discuss the construction of six- and four-dimensional Type IIB
orientifolds with vanishing untwisted RR tadpoles, but generically non-zero
twisted RR tadpoles. Tadpole cancellation requires the introduction of D-brane
systems with zero untwisted RR charge, but non-zero twisted RR charges. We
construct explicit models containing branes and antibranes at fixed points of
the internal space, or non-BPS branes partially wrapped on it. The models are
non-supersymmetric, but are absolutely stable against decay to supersymmetric
vacua. For particular values of the compactification radii tachyonic modes may
develop, triggering phase transitions between the different types of non-BPS
configurations of branes, which we study in detail in a particular example. As
an interesting spin-off, we show that the \IT^6/\IZ_4 orientifold without
vector structure, previously considered inconsistent due to uncancellable
twisted tadpoles, can actually be made consistent by introducing a set of
brane-antibrane pairs whose twisted charge cancels the problematic tadpole.Comment: 36 pages, Latex, 4 eps figure
Iron and Mechanisms of Neurotoxicity
The accumulation of transition metals (e.g., copper, zinc, and iron) and the dysregulation of their metabolism are a hallmark in the pathogenesis of several neurodegenerative diseases. This paper will be focused on the mechanism of neurotoxicity mediated by iron. This metal progressively accumulates in the brain both during normal aging and neurodegenerative processes. High iron concentrations in the brain have been consistently observed in Alzheimer's (AD) and Parkinson's (PD) diseases. In this connection, metalloneurobiology has become extremely important in establishing the role of iron in the onset and progression of neurodegenerative diseases. Neurons have developed several protective mechanisms against oxidative stress, among them, the activation of cellular signaling pathways. The final response will depend on the identity, intensity, and persistence of the oxidative insult. The characterization of the mechanisms mediating the effects of iron-induced increase in neuronal dysfunction and death is central to understanding the pathology of a number of neurodegenerative disorders
Quantum-ionic features in the absorption spectra of homonuclear diatomic molecules
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We show that additional features can emerge in the linear absorption spectra of homonuclear diatomic molecules when the ions are described quantum mechanically. In particular, the widths and energies of the peaks in the optical spectra change with the initial configuration, mass, and charge of the molecule. We introduce a model that can describe these features and we provide a quantitative analysis of the resulting peak energy shifts and width broadenings as a function of the mass.We acknowledge financial support from the European Research Council Advanced Grant DYNamo (Grant No. ERC-2010-AdG-267374), Spanish Grants No. FIS2013-46159-C3-1-P and No. PIB2010US-00652, and Grupo Consolidado UPV/EHU del Gobierno Vasco (Grant No. IT578-13). A.C.-U. acknowledges financial
support from the Departamento de Educacion, Universidades e Investigacion del Gobierno Vasco (Reference No. BFI-2011-26).Peer Reviewe
M5-brane geometries, T-duality and fluxes
We describe a duality relation between configurations of M5-branes in
M-theory and type IIB theory on Taub-NUT geometries with NSNS and RR 3-form
field strength fluxes. The flux parameters are controlled by the angles between
the M5-brane and the (T)duality directions. For one M5-brane, the duality leads
to a family of supersymmetric flux configurations which interpolates between
imaginary self-dual fluxes and fluxes similar to the Polchinski-Strassler kind.
For multiple M5-branes, the IIB configurations are related to fluxes for
twisted sector fields in orbifolds. The dual M5-brane picture also provides a
geometric interpretation for several properties of flux configurations (like
the supersymmetry conditions, their contribution to tadpoles, etc), and for
many non-trivial effects in the IIB side. Among the latter, the dielectric
effect for probe D3-branes is dual to the recombination of probe M5-branes with
background ones; also, a picture of a decay channel for non-supersymmetric
fluxes is suggested.Comment: 30 pages, 3 figure
Conceptual Design of a Single-Aisle Turboelectric Commercial Transport With Fuselage Boundary Layer Ingestion
A single-aisle commercial transport concept with a turboelectric propulsion system architecture was developed assuming entry into service in 2035 and compared to a similar technology conventional configuration. The turboelectric architecture consisted of two underwing turbofans with generators extracting power from the fan shaft and sending it to a rear fuselage, axisymmetric, boundary layer ingesting fan. Results indicate that the turbo- electric concept has an economic mission fuel burn reduction of 7%, and a design mission fuel burn reduction of 12% compared to the conventional configuration. An exploration of the design space was performed to better understand how the turboelectric architecture changes the design space, and system sensitivities were run to determine the sensitivity of thrust specific fuel consumption at top of climb and propulsion system weight to the motor power, fan pressure ratio, and electrical transmission efficiency of the aft boundary layer ingesting fan
Holographic dual of the Standard Model on the throat
We apply recent techniques to construct geometries, based on local Calabi-Yau
manifolds, leading to warped throats with 3-form fluxes in string theory, with
interesting structure at their bottom. We provide their holographic dual
description in terms of RG flows for gauge theories with almost conformal
duality cascades and infrared confinement. We describe a model of a throat with
D-branes at its bottom, realizing a 3-family Standard Model like chiral sector.
We provide the explicit holographic dual gauge theory RG flow, and describe the
appearance of the SM degrees of freedom after confinement. As a second
application, we describe throats within throats, namely warped throats with
discontinuous warp factor in different regions of the radial coordinate, and
discuss possible model building applications.Comment: 46 pages, 21 figures, reference adde
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