Comments on Orientifolds without Vector Structure

TL;DR

This paper explores type I string compactifications lacking vector structure, clarifying their relation to discrete B-field flux and T-duality, and demonstrates the consistency of certain GUT models with discrete fluxes on Calabi-Yau manifolds.

Contribution

It provides a detailed analysis of type I compactifications without vector structure, linking Z2 obstructions to fluxes and extending results to Calabi-Yau orientifolds.

Findings

Relation of Z2 obstruction to discrete B-field flux clarified

Consistency of 3-generation GUT models with discrete flux shown

Extension of results to Calabi-Yau orientifolds demonstrated

Abstract

We revisit type I compactifications with a Spin(32)/Z2 gauge bundle that admits no vector structure. We elucidate the relation of this Z2 obstruction to discrete B-field flux and to 't Hooft flux and clarify some subtleties in the T-duality transformation to type IIA intersecting D-brane models. We reexamine the earliest 3-generation GUT model on magnetized D-branes and show its consistency when a discrete B-flux is switched on. We further generalize partially known results for toroidal models to type I compactifications without vector structure and their mirror dual type IIA orientifolds on genuine Calabi-Yau manifolds. We illustrate this by working out the example of the quintic in some detail.