O-folds: Orientifolds and Orbifolds in Exceptional Field Theory

TL;DR

This paper unifies the description of orientifolds and orbifolds in string and M-theory using exceptional field theory, enabling a comprehensive understanding of various theories and their symmetries through a single extended framework.

Contribution

It introduces a unified ExFT approach to describe orientifolds and orbifolds, including localised non-Abelian vectors, and reproduces known theories from a single formalism.

Findings

Single $ ext{Z}_2$ orbifold yields M-theory, type II, and heterotic theories.

Inclusion of localised non-Abelian vectors at fixed points.

Reproduction of gauge transformations, Bianchi identities, and actions.

Abstract

We describe conventional orientifold and orbifold quotients of string and M-theory in a unified approach based on exceptional field theory (ExFT). Using an extended spacetime, ExFT combines all the maximal ten and eleven dimensional supergravities into a single theory manifesting a global symmetry corresponding to the exceptional series of Lie groups. Here we will see how this extends to half-maximal theories by showing how a single $\mathbb{Z}_2$ generalised orbifold (or O-fold) of ExFT gives rise to M-theory on an interval, type II with orientifold planes and the heterotic theories in an elegant fashion. We study in more detail such orbifold and orientifold actions preserving half-maximal supersymmetry, and show how the half-maximal structure of ExFT permits the inclusion of localised non-Abelian vector multiplets located at the orbifold fixed points. This allows us to reproduce for the $\mathbb{Z}_2$ example the expected modifications to the gauge transformations, Bianchi identities and actions of the theories obtained from the single ExFT starting point. We comment on the prospects of studying anomaly cancellation and more complicated, non-perturbative O-folds in the ExFT framework.