The positive tropical Grassmannian, the hypersimplex, and the m=2 amplituhedron

Abstract

The positive Grassmannian [FIGURE] is a cell complex consisting of all points in the real Grassmannian whose Plücker coordinates are non-negative. In this paper we consider the image of the positive Grassmannian and its positroid cells under two different maps: the moment map μ onto the hypersimplex [31] and the amplituhedron map ˜Z onto the amplituhedron [6]. For either map, we define a positroid dissection to be a collection of images of positroid cells that are disjoint and cover a dense subset of the image. Positroid dissections of the hypersimplex are of interest because they include many matroid subdivisions; meanwhile, positroid dissections of the amplituhedron can be used to calculate the amplituhedron’s ‘volume’, which in turn computes scattering amplitudes in N = 4 super Yang-Mills. We define a map we call T-duality from cells of [FIGURE] to cells of [FIGURE] and conjecture that it induces a bijection from positroid dissections of the hypersimplex k +1, n to positroid dissections of the amplituhedron A n k ,2; we prove this conjecture for the (infinite) class of BCFW dissections. We note that T-duality is particularly striking because the hypersimplex is an (n − 1)-dimensional polytope while the amplituhedron A n k ,2 is a 2k-dimensional non-polytopal subset of the Grassmannian Gr k k +2. Moreover, we prove that the positive tropical Grassmannian is the secondary fan for the regular positroid subdivisions of the hypersimplex, and prove that a matroid polytope is a positroid polytope if and only if all 2D faces are positroid polytopes. Finally, toward the goal of generalizing T-duality for higher m, we define the momentum amplituhedron for any even m.