Probing Scalar Effective Field Theories with the Soft Limits of Scattering Amplitudes

Understand

We investigate the soft behaviour of scalar effective field theories (EFTs) when there is a number of distinct derivative power counting parameters, $\rho_1< \rho_2<\ldots < \rho_Q$.

• We clarify the notion of an enhanced soft limit and use these to extend the scope of on-shell recursion techniques for scalar EFTs.
• As an example, we perform a detailed study of theories with two power counting parameters, $\rho_1=1$ and $\rho_2=2$, that include the shift symmetric generalised galileons.
• We demonstrate that the minimally enhanced soft limit uniquely picks out the Dirac-Born-Infeld (DBI) symmetry, including DBI galileons.