Double-Soft Limits of Gluons and Gravitons

Understand

The double-soft limit of gluon and graviton amplitudes is studied in four dimensions at tree level.

• In general this limit is ambiguous and we introduce two natural ways of taking it: A consecutive double-soft limit where one particle is taken soft before the other and a simultaneous limit where both particles are taken soft uniformly.
• All limits yield universal factorisation formulae which we establish by BCFW recursion relations down to the subleading order in the soft momentum expansion.
• These formulae generalise the recently discussed subleading single-soft theorems.