A note on polytopes for scattering amplitudes

Abstract

In this note we continue the exploration of the polytope picture for scattering amplitudes, where amplitudes are associated with the volumes of polytopes in generalized momentum-twistor spaces. After a quick warm-up example illustrating the essential ideas with the elementary geometry of polygons in $\mathbb{C}{\mathbb{P}^{{2}}}$, we interpret the 1-loop MHV integrand as the volume of a polytope in $\mathbb{C}{\mathbb{P}^{{3}}}$ × $\mathbb{C}{\mathbb{P}^{{3}}}$, which can be thought of as the space obtained by taking the geometric dual of the Wilson loop in each $\mathbb{C}{\mathbb{P}^{{3}}}$ of the product. We then review the polytope picture for the NMHV tree amplitude and give it a more direct and intrinsic definition as the geometric dual of a canonical “square” of the Wilson-Loop polygon, living in a certain extension of momentum-twistor space into $\mathbb{C}{\mathbb{P}^{{4}}}$. In both cases, one natural class of triangulations of the polytope produces the BCFW/CSW representations of the amplitudes; another class of triangulations leads to a striking new form, which is both remarkably simple as well as manifestly cyclic and local.