Gleaning gravitational amplitudes -- a double copy for canceling dilatons

Abstract

Scattering amplitudes in general relativity can be conveniently computed using the double copy, which relates them to Yang-Mills amplitudes. However, unwanted dilatons are sourced by massive scalar matter, which must be removed from the double copy in order to match the long range gravitational interactions. In this paper, we study how to automatically cancel out the dilatons by finding a suitable double-copy prescription in terms of gauge-theory fields, effectively treating the new contributions as ghosts that subtract out the unwanted states. At tree level, we find that an asymmetric double copy can reproduce the dilaton graphs in general dimension, which we explicitly verify up to six external massive scalars. Considering a one-loop four-point example, the same asymmetric double copy needs to be supplemented by the subtraction of bubble graphs that originate both from the axion and a residual dilaton term.