Correzioni di soglia in modelli di brane ad angolo

As a candidate theory of both gauge and gravitational forces, String Theory must be able to reproduce the Standard Model particles and interactions in the low energy limit. In this thesis I present the necessary tools to construct vacuum configurations with chiral matter and unitary gauge groups in the context of unoriented superstrings: open descendants construction, orbifold compactification, and magnetized D-branes (together with their corresponding T-dual version in terms of branes at angles). Moreover, I discuss the method to compute threshold corrections in these theories; these are one-loop corrections to the gauge coupling constants which arise after integrating out the massive string modes. Threshold corrections are necessary to determine gauge coupling uni?cation at the Planck mass scale. As an illustrative example I present an exhaustive study of T4/ZN six-dimensional models with minimal supersymmetry: even though they obviously do not represent phenomenologically viable models, they are useful benchmarks to understand the basic concepts of the general construction. In particular, I show how massless spectra and threshold corrections can be obtained from one-loop vacuum amplitudes. Finally, I generalize the T4/Z2 model by allowing the possibility of several stacks of rotated D-branes (in the T-dual description). This is interesting because it permits to obtain replications of chiral matter and unitary gauge groups.