Type II strings are Exceptional

TL;DR

This paper introduces the exceptional sigma model, a two-dimensional string theory formulated with exceptional field theory symmetries, unifying supergravity fields and relating to known string actions without relying on supersymmetry.

Contribution

It constructs a manifestly $E_{D(D)}$-covariant sigma model using exceptional field theory, uniquely fixing the Lagrangian and connecting to type IIA and IIB string actions.

Findings

Provides a Weyl-invariant Lagrangian fixed by local symmetries

Relates the model to standard type IIA and IIB string actions

Predicts correct gauge field couplings without supersymmetry

Abstract

We construct the exceptional sigma model: a two-dimensional sigma model coupled to a supergravity background in a manifestly (formally) $E_{D(D)}$-covariant manner. This formulation of the background is provided by Exceptional Field Theory (EFT), which unites the metric and form fields of supergravity in $E_{D(D)}$ multiplets before compactification. The realisation of the local symmetries of EFT on the worldsheet uniquely fixes the Weyl-invariant Lagrangian and allows us to relate our action to the usual type IIA fundamental string action and a form of the type IIB $(m,n)$ action. This uniqueness "predicts" the correct form of the couplings to gauge fields in both Neveu-Schwarz and Ramond sectors, without invoking supersymmetry.