Nonperturbative negative geometries: amplitudes at strong coupling and the amplituhedron

Abstract

The amplituhedron determines scattering amplitudes in planar mathcal{N} = 4 super Yang-Mills by a single “positive geometry” in the space of kinematic and loop variables. We study a closely related definition of the amplituhedron for the simplest case of four-particle scattering, given as a sum over complementary “negative geometries”, which provides a natural geometric understanding of the exponentiation of infrared (IR) divergences, as well as a new geometric definition of an IR finite observable mathcal{F} (g, z) — dually interpreted as the expectation value of the null polygonal Wilson loop with a single Lagrangian insertion — which is directly determined by these negative geometries. This provides a long-sought direct link between canonical forms for positive (negative) geometries, and a completely IR finite post-loop-integration observable depending on a single kinematical variable z, from which the cusp anomalous dimension Γcusp(g) can also be straightforwardly obtained. We study an especially simple class of negative geometries at all loop orders, associated with a “tree” structure in the negativity conditions, for which the contributions to mathcal{F} (g, z) and Γcusp can easily be determined by an interesting non-linear differential equation immediately following from the combinatorics of negative geometries. This lets us compute these “tree” contributions to mathcal{F} (g, z) and Γcusp for all values of the ‘t Hooft coupling. The result for Γcusp remarkably shares all main qualitative characteristics of the known exact results obtained using integrability.