Soft Theorems from String Theory

TL;DR

This work demonstrates how soft limits of closed-string amplitudes can be derived within bosonic string theory, extending known results to dilaton and Kalb-Ramond emissions and establishing a universal single-soft operator S that acts on n-point amplitudes to produce the n+1-point result. It further analyzes the leading double-soft behavior of gluon amplitudes on Dp-branes, expressing the result as a gauge-invariant convolution with explicit soft factors, including cases where soft gluons bracket a hard particle. The findings reproduce established graviton soft theorems, reveal distinct structures for the dilaton and Kalb-Ramond, and show α' independence of soft limits, while offering a clear path to higher-order soft terms and extensions to superstrings. Overall, the paper positions string theory as a robust, dimension-independent tool for deriving low-energy theorems in gauge and gravity theories and informs future applications to more complex amplitudes.

Abstract

Soft behaviour of closed string amplitudes involving dilatons, gravitons and anti-symmetric tensors, is studied in the framework of bosonic string theory. The leading double soft limit of gluons is analysed as well, starting from scattering amplitudes computed in the open bosonic string. Field theory expressions are then obtained by sending the string tension to infinity. The presented results have been derived in the papers of Ref.[1].