Related papers: On the stability of open-string orbifold models wi…
We compute at one loop the two-point functions of massless scalars in the Neumann-Dirichlet open-string sector of the type IIB orientifold compactified on $T^2\times T^4/\mathbb{Z}_2$, when $\mathcal{N}=2$ supersymmetry is spontaneously…
We discuss tachyon-free examples of (Type IIB on) non-compact non-supersymmetric orbifolds. Tachyons are projected out by discrete torsion between orbifold twists, while supersymmetry is broken by a Scherk-Schwarz phase (+1/-1 when acting…
We show how to construct chiral tachyon-free perturbative orientifold models, where supersymmetry is broken at the string scale on a collection of branes while, to lowest order, the bulk and the other branes are supersymmetric. In higher…
We study supersymmetry breaking by Scherk-Schwarz compactifications in type I string theory. While in the gravitational sector all mass splittings are proportional to a (large) compactification radius, supersymmetry remains unbroken for the…
We discuss the possibility of finding scenarios, within type IIB string theory compactified on Calabi-Yau orientifolds with fluxes, for realizing gauge mediated supersymmetry breaking. We find that while in principle such scenarios are not…
When supersymmetry is spontaneously broken at tree level, the spectrum of the heterotic string compactified on orbifolds of tori contains an infinite number of potentially tachyonic modes. We show that this implies instabilities of…
We present a Z_2 x Z_2 four dimensional orientifold in which supersymmetry is broken by a temperature-like Scherk-Schwarz mechanism. As usual in this type of models, at the tree-level the breaking affects only the branes tangent to the…
In this paper we discuss compactifications of type II superstrings where the moduli of the internal Calabi-Yau space vary over four-dimensional space time. The corresponding solutions of four-dimensional N=2 supergravity are given by…
We show that there exist supersymmetric Minkowski vacua on Type IIB toroidal orientifold with general flux compactifications where the RR tadpole cancellation conditions can be relaxed elegantly. Then we present a realistic Pati-Salam like…
We analyse a class of four-dimensional heterotic ground states with N=2 space-time supersymmetry. From the ten-dimensional perspective, such models can be viewed as compactifications on a six-dimensional manifold with SU(2) holonomy, which…
The first part of this review tries to provide a self-contained view of supersymmetry breaking from the bottom-up perspective. We thus describe N=1 supersymmetry in four dimensions, the Standard Model and the MSSM, with emphasis on the…
We consider 5D supersymmetric black holes in string theory compactifications that partially break supersymmetry. We compactify type IIB on $T^4$ and then further compactify on a circle with a duality twist to give Minkowski vacua preserving…
We study a class of flux compactifications that have all the moduli stabilised, a high (GUT) string scale and a low (TeV) gravitino mass that is generated dynamically. These non-geometric compactifications correspond to type II string…
We consider the compactification of type IIB superstring theory on K3 $\times$ K3. We obtain the massless spectrum of the resulting two dimensional theory and show that the model is free of gravitational anomaly. We then consider an…
We analyze non-supersymmetric four dimensional open string models of type IIB string theory compactified on $T^2\times K3$ with Scherk-Schwarz deformation acting on an $S^1$ of the $T^2$ torus. We find that there are always two solutions to…
We show that there exist supersymmetric Minkowski vacua on Type IIB toroidal orientifold with general flux compactifications where the RR tadpole cancellation conditions can be relaxed and the Freed-Witten anomaly can be cancelled…
We study a supersymmetric field theory in six dimensions compactified on the orbifold T^2/Z_2 with two Wilson lines. After supersymmetry breaking, the Casimir energy fixes the shape moduli at fixed points in field space where the symmetry…
This thesis contains an introductory chapter on orbifolds. The following chapter explains the foundations of orientifolds. Chapters 4-7 present own research. In chapter 4 we quantize open strings with linear boundary conditions, as they…
The study of string models including both unoriented closed strings and open strings presents a number of new features when compared to the standard case of models of oriented closed strings only. We review some basic features of the…
We propose examples, which involve orbifolds by elements of the U-duality group, with M-theory moduli fixed at the eleven-dimensional Planck scale. We begin by reviewing asymmetric orbifold constructions in perturbative string theory, which…