578 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
Securitizing Immigrants: Applying Securitization Theory in German Politics
This manuscript demonstrates how the use of securitization by the German political party the Alternative für Deutschland (AfD) has gained them votes in the German federal elections. The securitization focused on the refugee crisis and the effects that the refugees would have on Germany and its citizens. While mainstream German political parties adopted a neutral stance towards the crisis, the AfD separated themselves by adopting a strong anti-immigrant stance. The concept of securitization has not been fully applied to the German political parties. As a proxy for the political party, the paper analyzes the policy platforms and statements regarding immigration, designed to gain popularity and votes. In order to do this, the paper first defines securitization and then analyzes a variety of sources, including the political parties’ manifestos, in order to show how they have developed and changed their political agendas and beliefs between the years of 2013-2019. This paper compares voting polls and statistics to examine how the party’s use of securitization has garnered them popularity and votes and to find which groups tend to vote for them. The research showed that the party’s shift to securitizing the refugee crisis resulted in the increase of votes in the German federal elections. The AfD placed a sizable focus on their campaign towards immigration after the beginning of the crisis in 2015. For the AfD whose whole campaign focused on immigration, it saw a huge boost of votes during the 2017 German Federal election, managing to reach third place in the number of votes it received
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
Stable Non-BPS States and Their Holographic Duals
Stable non-BPS states can be constructed and studied in a variety of contexts
in string theory. Here we review some interesting constructions that arise from
suspended and wrapped branes. We also exhibit some stable non-BPS states that
have holographic duals.Comment: 10 pages, LaTeX, 10 .eps figures (included); based on a talk given at
Strings 2000, Michiga
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
Timing the moment when atom decays (and Schroedinger's cat dies)
We propose detecting the moment an atom emits a photon by means of a nearly
classical macroscopic clock and discuss its viability. It is shown that what
happens in such a measurement depends on the relation between the clock's
accuracy and the width of the energy range available to the photon.
Implications of the analysis for the long standing Schroedinger's cat problem
are reported.Comment: 21 pages, 5 figure
Computational Assessment of the Benefits of Boundary Layer Ingestion for the D8 Aircraft
To substantially reduce the fuel burn of future commercial transportation aircraft, the boundary layer ingestion idea is investigated. The idea is that an engine placed in the wake of the aircraft it is propelling is more efficient than a conventional engine placement under the wing or on pods mounted to the rear of the fuselage. The top, rear of the fuselage is thus designed to act as a diffuser such that the engines can be placed there with a minimal nacelle. The boundary layer thickens over the rear of the fuselage such that a large portion of it is ingested by the fan. To assess whether the boundary layer ingesting (BLI) engine placement is indeed advantageous, a study of the nacelle aerodynamics is carried out using Overflow, a viscous CFD flow solver that uses overset meshes. The computed forces and moments are compared to a wind tunnel experiment for validation. Some aspects of the design are verified using the simulation results. Finally, the effect of the nacelle placement is assessed by comparing the BLI nacelle configuration to a podded nacelle configuration and to the unpowered (without nacelles) aircraft
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
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