The Coulomb-branch S-matrix from massless amplitudes

Abstract

We present a systematic method to extract the entire tree-level S-matrix on the Coulomb branch of N=4 SYM from soft-scalar limits of on-shell amplitudes at the origin of moduli space. Massive amplitudes in the spontaneously-broken theory can thus be computed from on-shell amplitudes in the massless, unbroken theory. To check this correspondence, we first prove that soft and collinear divergences in the required massless amplitudes cancel for a judicious choice of soft-scalar momenta. We then explicitly verify our proposal in examples with arbitrarily many external legs and to all orders in the mass. As a byproduct, the construction leads to a massive CSW-like expansion that reproduces several known all-n results for Coulomb-branch amplitudes in an effortless way. We briefly discuss the extension of our method to loop integrands.