Kappa-symmetry of superstring sigma model and generalized 10d supergravity equations

TL;DR

This paper explores how kappa-symmetry constrains 10d superstring backgrounds, leading to generalized supergravity equations that extend standard models and include new solutions related to T-duality and integrable deformations.

Contribution

It derives generalized supergravity equations from kappa-symmetry constraints, revealing new solutions and clarifying their relation to standard supergravity and T-duality.

Findings

Generalized supergravity equations depend on vectors X_a and K_a.

Standard supergravity equations are recovered when K_a=0 and X_a is a dilaton gradient.

New solutions involve backgrounds with isometries, related to integrable deformations.

Abstract

We determine the constraints imposed on the 10d target superspace geometry by the requirement of classical kappa-symmetry of the Green-Schwarz superstring. In the type I case we find that the background must satisfy a generalization of type I supergravity equations. These equations depend on an arbitrary vector X_a and imply the one-loop scale invariance of the GS sigma model. In the special case when X_a is the gradient of a scalar \phi (dilaton) one recovers the standard type I equations equivalent to the 2d Weyl invariance conditions of the superstring sigma model. In the type II case we find a generalized version of the 10d supergravity equations the bosonic part of which was introduced in arXiv:1511.05795. These equations depend on two vectors \X_a and K_a subject to 1st order differential relations (with the equations in the NS-NS sector depending only on the combination X_a = \X_a + K_a). In the special case of K_a=0 one finds that \X_a=\d_a \phi and thus obtains the standard type II supergravity equations. New generalized solutions are found if K_a is chosen to be a Killing vector (and thus they exist only if the metric admits an isometry). Non-trivial solutions of the generalized equations describe K-isometric backgrounds that can be mapped by T-duality to type II supergravity solutions with dilaton containing a linear isometry-breaking term. Examples of such backgrounds appeared recently in the context of integrable \eta-deformations of AdS_n x S^n sigma models. The classical kappa-symmetry thus does not, in general, imply the 2d Weyl invariance conditions for the GS sigma model (equivalent to type II supergravity equations) but only weaker scale invariance type conditions.