Soft behavior of a closed massless state in superstring and universality in the soft behavior of the dilaton

TL;DR

This paper analyzes tree-level scattering amplitudes in superstring theory, revealing universal and correction-free soft theorems for gravitons and dilatons, extending previous field theory and bosonic string results.

Contribution

It derives soft theorems for massless states in superstring theory, showing universality and absence of string corrections at subsubleading order, especially for the dilaton.

Findings

Soft theorems factorize amplitudes into soft and hard parts.

Superstring soft theorems for gravitons lack string corrections at subsubleading order.

Dilaton soft theorem is universally free of string corrections in all string theories.

Abstract

We consider the tree-level scattering amplitudes in the NS-NS (Neveu-Schwarz) massless sector of closed superstrings in the case where one external state becomes soft. We compute the amplitudes generically for any number of dimensions and any number and kind of the massless closed states through the subsubleading order in the soft expansion. We show that, when the soft state is a graviton or a dilaton, the full result can be expressed as a soft theorem factorizing the amplitude in a soft and a hard part. This behavior is similar to what has previously been observed in field theory and in the bosonic string. Differently from the bosonic string, the supersymmetric soft theorem for the graviton has no string corrections at subsubleading order. The dilaton soft theorem, on the other hand, is found to be universally free of string corrections in any string theory.