BMS, String Theory, and Soft Theorems

Abstract

We study the action of the BMS group in critical, bosonic string theory living on a target space of the form $\mathbb{M}^{d}\times C$. Here $M^{d}$ is $d$-dimensional (asymptotically) flat spacetime and $C$ is an arbitrary compactification. We provide a treatment of generalized Ward--Takahashi identities and derive consistent boundary conditions for any $d$ from string theory considerations. Finally, we derive BMS transformations in higher dimensional spacetimes and show that the generalized Ward--Takahashi identity of BMS produces Weinberg's soft theorem in string theory.