Orientifolds of Gepner Models

We systematically construct and study Type II Orientifolds based on Gepner models which have N=1 supersymmetry in 3+1 dimensions. We classify the parity symmetries and construct the crosscap states. We write down the conditions that a configuration of rational branes must satisfy for consistency (tadpole cancellation and rank constraints) and spacetime supersymmetry. For certain cases, including Type IIB orientifolds of the quintic and a two parameter model, one can find all solutions in this class. Depending on the parity, the number of vacua can be large, of the order of 10^{10}-10^{13}. For other models, it is hard to find all solutions but special solutions can be found -- some of them are chiral. We also make comparison with the large volume regime and obtain a perfect match. Through this study, we find a number of new features of Type II orientifolds, including the structure of moduli space and the change in the type of O-planes under navigation through non-geometric phases.Comment: 142 page

Intersecting Brane Worlds -- A Path to the Standard Model ?

In this review we describe the general geometrical framework of brane world constructions in orientifolds of type IIA string theory with D6-branes wrapping 3-cycles in a Calabi-Yau 3-fold. These branes generically intersect in points on the internal space, and the patterns of intersections govern the chiral fermion spectra. We discuss how the open string spectra in intersecting brane models are constructed, how the Standard Model can be embedded, and also how supersymmetry can be realized in this class of string vacua. After the general considerations we specialize the discussion to the case of orbifold backgrounds with intersecting D6-branes and to the quintic Calabi-Yau manifold. Then, we discuss parts of the effective action of intersecting brane world models. Specifically we compute from the Born-Infeld action of the wrapped D-branes the tree-level, D-term scalar potential, which is important for the stability of the considered backgrounds as well as for questions related to supersymmetry breaking. Second, we review the recent computation concerning of gauge coupling unification and also of one-loop gauge threshold corrections in intersecting brane world models. Finally we also discuss some aspects of proton decay in intersecting brane world models.Comment: 31 pages, To appear in the proceedings of the RTN-workshop ``The quantum structure of spacetime and the geometric nature of fundamental interactions'', September 2003 in Copenhagen, revised version contains new refs and one corrected equatio