(8,0) Quantum mechanics and symmetry enhancement in type I' superstrings

In string theory, black holes can be made from brane systems that are partially delocalized in string theory's extra dimensions. It is expected that they correspond to an ensemble of pure, horizonless microstates sourced by localized versions of this brane system. This thesis aims at studying the relationship between supersymmetric black holes and their microstates. In particular, we apply a concept called local supersymmetry enhancement to 1/4-BPS and 1/8-BPS systems that co...

We present special classes of orientifold models involving supersymmetry breaking via branes at angles. Type II superstring theories are compactified on a two torus times a four-dimensional orbifold. Combining worldsheet parity with a reflection of half of the compact coordinates leads to D6-branes at angles which are mapped onto each other by the orbifold group, while applying the geometric action only along one coordinate leads to intersecting D8-branes with non-trivial tra...

We consider solutions of massive IIA supergravity corresponding to the half-BPS intersection of D8/D4-branes with fundamental strings. The $1+1$-dimensional intersection preserves the symmetry $D(2,1;\gamma;1) \times SO(4)$. We give a reduction and partial integration of the BPS equations for this symmetry group. We then specialize to the cases of enhanced supersymmetry corresponding to $\gamma = -1/2,-2$ or $\gamma = 1$. In the first case, we show that the only solution with...

This report presents some studies of the gauge/string theory correspondence, a deep relation that is possible to establish between quantum field theories with local gauge symmetry and superstring theories including gravity. In its original version, known as AdS/CFT duality, the correspondence involves N=4 Super Yang-Mills theory in four space-time dimensions, which is a superconformal theory with a high degree of supersymmetry, thus very far from describing the physical world...

In this paper we consider compactifications of type I strings on Abelian orbifolds. We discuss the tadpole cancellation conditions for the general case with D9-branes only. Such compactifications have (perturbative) heterotic duals which are also realized as orbifolds (with non-standard embedding of the gauge connection). The latter have extra twisted states that become massive once orbifold singularities are blown-up. This is due to the presence of perturbative heterotic sup...

Starting from a peculiar orientifold projection proposed long ago by Angelantonj and Cardella, we elaborate on a novel perturbative scenario that involves only D-branes, together with the two types of orientifold planes ${\rm O}_{\pm}$ and anti-orientifold planes $\overline{{\rm O}}_{\pm}$. We elucidate the microscopic ingredients of such models, connecting them to a novel realization of brane supersymmetry breaking. Depending on the position of the D-branes in the internal s...

This introductory part of the author's PhD compilation thesis discusses non-perturbative aspects of string theory, with focus on D-instantons which play key roles in the context of string phenomenology and dualities. By translating ideas from ordinary instanton calculus in field theory, D-instanton calculus is formulated and applied to supersymmetric gauge theories realized as world volume theories of spacefilling D-branes in non-trivial backgrounds. We show that, for configu...

We extend the study on D-branes in the type IIB plane wave background to less supersymmetric configurations. We show that many new supersymmetric D-branes can be found by turning on electric as well as magnetic background fluxes, or constantly boosting D-branes.

A class of D-branes for the type IIB plane-wave background is considered that preserve half the dynamical supersymmetries of the light-cone gauge. The D-branes of this type are the euclidean (or instantonic) (0,0), (0,4) and (4,0) branes (where (r,s) denotes a brane oriented with r axes in the first four directions transverse to the +,- light-cone and s axes in the second four directions). Corresponding lorentzian D-branes are (+,-;0,0), (+,-;0,4) and (+,-;4,0). These are con...

In this thesis, we study aspects of D-brane realizations of the Standard Model. Specifically, we study orientifold models with rotation and translation elements that break supersymmetry, provide the general consistency conditions and derive the massless spectrum for these type of orientifolds. These models contain in general anomalous U(1) gauge fields. The Green-Schwarz mechanism cancels the anomaly and provides a mass term for the anomalous gauge fields. We calculate the ba...