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

Imaginary past of a quantum particle moving on imaginary time

The analytical continuation of classical equations of motion to complex times suggests that a tunneling particle spends in the barrier an imaginary duration ⁢|\mathcal{T}|. Does this mean that it takes a finite time to tunnel, or should tunneling be seen as an instantaneous process? It is well known that examination of the adiabatic limit in a small additional AC field points towards |\mathcal{T}| being the time it takes to traverse the barrier. However, this is only half the story. We probe the transmitted particle's history, and find that it remembers very little of the field's past behavior, as if the transit time were close to zero. The ensuing contradiction suggests that the question is ill posed, and we explain why

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