Subsubleading soft theorems of gravitons and dilatons in the bosonic string

TL;DR

This paper derives subsubleading soft theorems for gravitons and dilatons in bosonic string theory, revealing string corrections for gravitons but not for dilatons, and connects these results to gauge invariance and conformal transformations.

Contribution

It provides the first derivation of subsubleading soft theorems in bosonic string theory, including string corrections, and links them to gauge invariance and conformal symmetry.

Findings

String corrections appear in soft graviton theorems but vanish for dilatons.

Soft theorems can be derived from three-point string vertices and exchange diagrams.

The dilaton soft theorem's non-singular part generates space-time special conformal transformations.

Abstract

Starting from the amplitude with an arbitrary number of massless closed states of the bosonic string, we compute the soft limit when one of the states becomes soft to subsubleading order in the soft momentum expansion, and we show that when the soft state is a graviton or a dilaton, the full string amplitude can be expressed as a soft theorem through subsubleading order. It turns out that there are string corrections to the field theoretical limit in the case of a soft graviton, while for a soft dilaton the string corrections vanish. We then show that the new soft theorems, including the string corrections, can be simply obtained from the exchange diagrams where the soft state is attached to the other external states through the three-point string vertex of three massless states. In the soft-limit, the propagator of the exchanged state is divergent, and at tree-level these are the only divergent contributions to the full amplitude. However, they do not form a gauge invariant subset and must be supplemented with extra non-singular terms. The requirement of gauge invariance then fixes the complete amplitude through subsubleading order in the soft expansion, reproducing exactly what one gets from the explicit calculation in string theory. From this it is seen that the string corrections at subsubleading order arise as a consequence of the three-point amplitude having string corrections in the bosonic string. When specialized to a soft dilaton, it remarkably turns out that the string corrections vanish and that the non-singular piece of the subsubleading term of the dilaton soft theorem is the generator of space-time special conformal transformation.