Notes on Scattering Amplitudes as Differential Forms

TL;DR

This paper introduces differential forms on spinor variables for gauge theories, revealing a unified geometric structure for scattering amplitudes in ${ m N}=4$ SYM, and connects it to the amplituhedron and twistor-string theory.

Contribution

It develops a new geometric framework using differential forms in spinor space for scattering amplitudes, extending the amplituhedron concept beyond momentum twistor space.

Findings

Tree-level amplitudes form a $d\,log$ structure in spinor variables.

All-multiplicity MHV and NMHV forms are explicitly derived.

Planar loop integrands can be expressed as $d\,log$ forms, suggesting a broader geometric picture.

Abstract

Inspired by the idea of viewing amplitudes in ${\cal N}=4$ SYM as differential forms on momentum twistor space, we introduce differential forms on the space of spinor variables, which combine helicity amplitudes in any four-dimensional gauge theory as a single object. In this note we focus on such differential forms in ${\cal N}=4$ SYM, which can also be thought of as "bosonizing" superamplitudes in non-chiral superspace. Remarkably all tree-level amplitudes in ${\cal N}=4$ SYM combine to a $d\log$ form in spinor variables, which is given by pushforward of canonical forms of Grassmannian cells, the tree forms can also be obtained using BCFW or inverse-soft construction, and we present all-multiplicity expression for MHV and NMHV forms to illustrate their simplicity. Similarly all-loop planar integrands can be naturally written as $d\log$ forms in the Grassmannian/on-shell-diagram picture, and we expect the same to hold beyond the planar limit. Just as the form in momentum twistor space reveals underlying positive geometry of the amplituhedron, the form in terms of spinor variables strongly suggests an "amplituhedron in momentum space". We initiate the study of its geometry by connecting it to the moduli space of Witten's twistor-string theory, which provides a pushforward formula for tree forms in ${\cal N}=4$ SYM.