Flavour-kinematics duality for Goldstone modes

Abstract

Three scalar effective field theories have special properties in terms of non-linear symmetries, soft limits and on-shell constructability that arise from their Goldstone nature: the non-linear $\sigma$-model, multi-DBI theory and the special Galileon. We discuss how these theories are related via flavour-kinematics duality, analogous to the colour-kinematics duality between gravity and gauge theories. At the off-shell level, we identify a specific mapping between the three theories that is crucially dependent on their non-linear symmetries. Similarly, we demonstrate how the on-shell amplitudes factorise into BCJ numerators describing flavour and a scalar version of kinematics, naturally leading to the inclusion of graviton exchange in the $SO(M,N)$ non-linear $\sigma$-model. Finally, we map those numerators onto each other, and comment on a similar relation to tensor kinematics. Our results highlight a common structure that underlies the physics of different Goldstone modes.