Interactions in D-brane configurations
We explore various interactions in D-brane configurations and their implications for the construction of phenomenologically viable string models. Initially, we investigate the scenario of parallel and perpendicular stacks of D-branes located on an orbifold singularity, as in the bottom-up construction. A supersymmetric D-brane model is presented, that has CP spontaneously broken by discrete torsion. The low energy physics is largely independent of the compactification scheme and the kahler metric has 'texture zeros’ dictated by the choice of discrete torsion. This motivates a simple ansatz for the kahler metric which results in a CKM matrix given in terms of two free parameters, hence we predict a single mixing angle and the CKM phase. The CKM phase is predicted to be close to /3.We then proceed to a discussion of a different class of models involving D-branes intersecting at arbitrary angles, and wrapping a compact internal space. Here we calculate tree level three and four point scattering amplitudes in type II string models with matter fields localised at D-brane intersections. We treat both the classical and quantum parts in detail, with the latter being computed using conformal field theory techniques developed for closed strings on orbifolds. Contributions from string states wrapping the internal space are also included. These calculations are then generalised to TV-point amplitudes, which are determined completely. Finally, we consider the application of these results to four fermion interactions. In particular, the implications for Higgs exchange in intersecting brane models is discussed.