The B-field soft theorem and its unification with the graviton and dilaton

Understand

In theories of Einstein gravity coupled with a dilaton and a two-form, a soft theorem for the two-form, known as the Kalb-Ramond B-field, has so far been missing.

• In this work we fill the gap, and in turn formulate a unified soft theorem valid for gravitons, dilatons and B-fields in any tree-level scattering amplitude involving the three massless states.
• The new soft theorem is fixed by means of on-shell gauge invariance and enters at the subleading order of the graviton's soft theorem.
• In contrast to the subsubleading soft behavior of gravitons and dilatons, we show that the soft behavior of B-fields at this order cannot be fully fixed by gauge invariance.