U-folds as K3 fibrations

TL;DR

This paper explores flux vacua in type IIB string theory with a focus on Calabi-Yau threefolds that are K3 fibrations, including geometric and non-geometric solutions with T-duality monodromies.

Contribution

It provides a detailed analysis of non-perturbative flux vacua using K3 fibrations and constructs explicit examples of geometric and non-geometric solutions with T-duality features.

Findings

Explicit examples of flux vacua with K3 fibrations.

Construction of non-geometric T-fold solutions.

Identification of T-duality monodromies exchanging 3- and 7-branes.

Abstract

We study N=2 four-dimensional flux vacua describing intrinsic non-perturbative systems of 3 and 7 branes in type IIB string theory. The solutions are described as compactifications of a G(ravity) theory on a Calabi Yau threefold which consists of a fibration of an auxiliary K3 surface over an S^2 base. In the spirit of F-theory, the complex structure of the K3 surface varying over the base codifies the details of the fluxes, the dilaton and the warp factors in type IIB string theory. We discuss in detail some simple examples of geometric and non-geometric solutions where the precise flux/geometry dictionary can be explicitly worked out. In particular, we describe non-geometric T-fold solutions exhibiting non-trivial T-duality monodromies exchanging 3- and 7-branes.