Perturbative and non-perturbative aspects of orientifold compactifications

This thesis mainly focuses on novel stringy instanton effects in orientifold compactifications. Such effects are known to generate superpotential couplings which are perturbatively forbidden due to U(1) selection rules arising from the Green-Schwarz mechanism. We discuss their gauge transformation behavior and zero mode structure and give an outline of how to compute these non-perturbative effects. Here we focus for concreteness on type IIA compactifications, where the instantons are given by E2-instantons, wrapping a three-cycle in the internal manifold and are pointlike in the four-dimensional spacetime.;Later we embed E2-instantons in local setups and give various examples of instanton induced superpotential couplings which are otherwise forbidden. In all these configurations non-perturbative effects give a natural explanation for hierarchies which are only poorly understood from the field theoretical point of view. Furthermore, we explicitly compute the instanton induced mass term of Majorana masses for right-handed neutrinos by using conformal field theory techniques.;Finally we investigate under what circumstances instantons provide the correct zero mode structure to induce superpotential corrections. We analyze two different strategies to lift additional undesired instanton zero modes. First we discuss the process of instanton recombination, and show that generically one needs additional instantons to generate superpotential couplings. With these multi-instantons we show that the class of superpotential contributing BPS instantons is much richer than mostly considered. Later we study the lifting of undesired zero modes in the type IIB framework with supersymmetric background flux. We show that indeed in certain situations the background fluxes lift the zero modes and the instanton gives rise to superpotential contributions.;Apart from the non-perturbative effects, we analyze the proton decay via dimension six operators in supersymmetric SU(5)-Grand unified models based on intersecting D6-branes. We include in addition to 10*1010*10 interactions also operators arising from 5¯*5¯10*10 interactions. In the course of that analysis we provide a detailed construction of vertex operators for any massless string excitation in an arbitrary intersecting brane configuration. With the knowledge of the vertex operators for the 10 and 5¯ chiral superfields, we explicitly calculate the string theory correlation functions for above operators. We show that for the most symmetric configuration, the stringy contribution to the proton life time is tSTp = (0.6--2.6) x 1036 years, which could be up to a factor of two and a half shorter than that one predicted in field theory.