Orbifold compactification of heterotic E 8 × E ′ 8 string theory is a source for promising grand unified model building. It can accommodate the successful aspects of grand unification while avoiding problems like doublet-triplet splitting in the Higgs sector. Many of the phenomenological properties of the 4-dimensional effective theory find an explanation through the geometry of compact space and the location of matter and Higgs fields. These geometrical properties can be used as a guideline for realistic model building.

Citation for published item:p¤ orsteD F nd ronekerD qF nd hreyerD F @PHHIA 9upersymmetri xw orientifolds in Rh with hErnes t nglesF9D xuler physis fFD SWQ @IEPAF ppF IPUEISRF Further information on publisher's website: httpXGGdxFdoiForgGIHFIHITGHSSHEQPIQ@HHAHHTITEU Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract We construct orientifolds of type IIA string theory. The theory is compactified on a T 6 /Z N × Z M orbifold. In addition worldsheet parity in combination with a reflection of three compact directions is modded out. Tadpole cancellation requires to add D-6-branes at angles. The resulting four dimensional theories are N = 1 supersymmetric and non-chiral.

In this note we elaborate on various five dimensional contributions to the effective 4D cosmological constant in brane systems. In solutions with vanishing 5D cosmological constant we describe a non-local mechanism of cancellation of vacuum energy between the brane and the singularities. We comment on a hidden fine tuning which is implied by this observation.

We study stringy fluctuations as a source for corrections to the Wilson loop as obtained from the superstring on adS 5 × S 5 /N = 4 SYM correspondence. We give a formal expression in terms of determinants of two-dimensional operators for the leading order correction. 02/99

We point out several subtleties arising in brane-world scenarios of cosmological constant cancellation. We show that solutions with curvature singularities are inconsistent, unless the contribution to the effective four-dimentional cosmological constant of the physics that resolves the singularities is fine-tuned. This holds for both flat and curved branes. Irrespective of this problem, we then study an isolated class of flat solutions in models where a bulk scalar field with a vanishing potential couples to a 3-brane. We give an example where the introduction of a bulk scalar potential results in a nonzero cosmological constant. Finally we comment on the stability of classical solutions of the brane system with respect to quantum corrections.1 For reviews of the cosmological constant problem see [6,7,8].

Moduli spaces of conformal field theories corresponding to current-current deformations are discussed. For WZW models, CFT and sigma model considerations are compared. It is shown that current-current deformed WZW models have WZW-like sigma model descriptions with non-bi-invariant metrics, additional B-fields and a non-trivial dilaton.

We present supersymmetry breaking four dimensional orientifolds of type IIA strings. The compact space is a torus times a four dimensional orbifold. The orientifold group reflects one direction in each torus. RR tadpoles are cancelled by D6-branes intersecting at angles in the torus and in the orbifold. The angles are chosen such that supersymmetry is broken. The resulting four dimensional theories contain chiral fermions. The tadpole cancellation conditions imply that there are no non-abelian gauge anomalies. The models also contain anomaly-free U (1) factors.

We generalize a five dimensional black hole solution of low energy effective string theory to arbitrary constant spatial curvature. After interchanging the signature of time and radius we reduce the 5d solution to four dimensions and obtain that way a four dimensional isotropic cosmological space time. The solution contains a dilaton, modulus field and torsion. Several features of the solution are discussed.