Recent Progress in Intersecting D-brane Models

The aim of this article is to review some recent progress in the field of intersecting D-brane models. This includes the construction of chiral, semi-realistic flux compactifications, the systematic study of Gepner model orientifolds, the computation of various terms in the low energy effective action and the investigation of the statistics of solutions to the tadpole cancellation conditions.Comment: 13 pages, 1 figure, contribution to the proceedings of the 37. Symposium Ahrenshoop, 23-27 August 2004, Wernsdorf, Germany, v3: more refs. adde

Gauge Coupling Unification in F-theory GUT Models

We investigate gauge coupling unification for F-theory respectively Type IIB orientifold constructions of SU(5) GUT theories with gauge symmetry breaking via non-trivial hypercharge flux. This flux has the non-trivial effect that it splits the values of the three MSSM gauge couplings at the string scale, thus potentially spoiling the celebrated one-loop gauge coupling unification. It is shown how F-theory can evade this problem in a natural way.Comment: 4 pages, 3 figures, v2: clarifying comments and refs. added, v3: footnote [24] adde

Discrete Deformations in Type I Vacua

We study supersymmetric orientifolds where the world-sheet parity transformation is combined with a conjugation of some compact complex coordinates. We investigate their T-duality relation to standard orientifolds and discuss the origin of continuous and discrete moduli. In contrast to standard orientifolds, the antisymmetric tensor describes a continuous deformation, while the off-diagonal part of the metric is frozen to quantized values and is responsible for the rank reduction of the gauge group. We also give a geometrical interpretation of some recently constructed six-dimensional permutational orientifolds.Comment: 12 pages, TeX, harvmac, 4 figures, eq.(3.15) change

T-duality, Non-geometry and Lie Algebroids in Heterotic Double Field Theory

A number of issues in heterotic double field theory are studied. This includes the analysis of the T-dual configurations of a flat constant gauge flux background, which turn out to be non-geometric. Performing a field redefinition to a non-geometric frame, these T-duals take a very simple form reminiscent of the constant Q- and R-flux backgrounds. In addition, it is shown how the analysis of arXiv:1304.2784 generalizes to heterotic generalized geometry. For every field redefinition specified by an O(D,D+n) transformation, the structure of the resulting supergravity action is governed by the differential geometry of a corresponding Lie algebroid.Comment: 27 pages, 1 figure, v2: refs adde

Superconformal Field Theories for Compact Manifolds with Spin(7) Holonomy

We present a construction of superconformal field theories for manifolds with Spin(7) holonomy. Geometrically these models correspond to the realization of Spin(7) manifolds as anti-holomorphic quotients of Calabi-Yau fourfolds. Describing the fourfolds as Gepner models and requiring anomaly cancellation we determine the resulting Betti numbers of the Spin(7) superconformal field theory. As in the G_2 case, we find that the Gepner model and the geometric result disagree.Comment: 16 pages, TeX, harvmac, refs. adde