Aspects of stability and phenomenology in type IIA orientifolds with intersecting D6‐branes

Intersecting branes have been the subject of an elaborate string model building for several years. After a general introduction into string theory, this work introduces in detail the toroidal and \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$\mathbb{Z}_N$\end{document} ‐orientifolds. The picture involving D9‐branes with B‐fluxes is shortly reviewed, but the main discussion employs the T‐dual picture of intersecting D6‐branes. The derivation of the R‐R and NS‐NS tadpole cancellation conditions in the conformal field theory is shown in great detail. Various aspects of the open and closed chiral and non‐chiral massless spectrum are discussed, involving spacetime anomalies and the generalized Green‐Schwarz mechanism. An introduction into possible gauge breaking mechanisms is given, too. Afterwards, both = 1 supersymmetric and non‐supersymmetric approaches to low energy model building are treated. Firstly, the problem of complex structure instabilities in toroidal Ω R ‐orientifolds is approached by a \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$\mathbb{Z}_3$\end{document} ‐orbifolded model. In particular, a stable non‐supersymmetric standard‐like model with three fermion generations is discussed. This model features the standard model gauge groups at the same time as having a massless hypercharge, but possessing an additional global B ‐ L symmetry. The electroweak Higgs mechanism and the Yukawa couplings are not realized in the usual way. It is shown that this model descends naturally from a flipped SU (5) GUT model, where the string scale has to be at least of the order of the GUT scale. Secondly, supersymmetric models on the \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$\mathbb{Z}_4$\end{document} ‐orbifold are discussed, involving exceptional 3‐cycles and the explicit construction of fractional D‐branes. A three generation Pati‐Salam model is constructed as a particular example, where several brane recombination mechanisms are used, yielding non‐flat and non‐factorizable branes. This model even can be broken down to a MSSM‐like model with a massless hypercharge. Finally, the possibility that unstable closed and open string moduli could have played the role of the inflaton in the evolution of the universe is being explored. In the closed string sector, the important slow‐rolling requirement can only be fulfilled for very specific cases, where some moduli are frozen and a special choice of coordinates is taken. In the open string sector, inflation does not seem to be possible at all.