The hypersimplexΔk+1,n\Delta _{k+1,n}is the image of the positive GrassmannianGrk+1,n≥0Gr^{\geq 0}_{k+1,n}under the moment map. It is a polytope of dimensionn−1n-1inRn\mathbb {R}^n. Meanwhile, the amplituhedronAn,k,2(Z)\mathcal {A}_{n,k,2}(Z)is the projection of the positive GrassmannianGrk,n≥0Gr^{\geq 0}_{k,n}into the GrassmannianGrk,k+2Gr_{k,k+2}under a mapZ~\tilde {Z}induced by a positive matrixZ∈Matn,k+2>0Z\in Mat_{n,k+2}^{>0}. Introduced in the context ofscattering amplitudes, it is not a polytope, and has full dimension2k2kinsideGrk,k+2Gr_{k,k+2}. Nevertheless, there seem to be remarkable connections between these two objects viaT-duality, as conjectured by Łukowski, Parisi, and Williams [Int. Math. Res. Not. (2023)]. In this paper we use ideas from oriented matroid theory, total positivity, and the geometry of the hypersimplex and positroid polytopes to obtain a deeper understanding of the amplituhedron. We show that the inequalities cutting outpositroid polytopes—images of positroid cells ofGrk+1,n≥0Gr^{\geq 0}_{k+1,n}under the moment map—translate into sign conditions characterizing the T-dualGrasstopes—images of positroid cells ofGrk,n≥0Gr^{\geq 0}_{k,n}underZ~\tilde {Z}. Moreover, we subdivide the amplituhedron intochambers, just as the hypersimplex can be subdivided into simplices, with both chambers and simplices enumerated by the Eulerian numbers. We use these properties to prove the main conjecture of Łukowski, Parisi, and Williams [Int. Math. Res. Not. (2023)]: a collection of positroid polytopes is a tiling of the hypersimplex if and only if the collection of T-dual Grasstopes is a tiling of the amplituhedronAn,k,2(Z)\mathcal {A}_{n,k,2}(Z)for allZZ. Moreover, we prove Arkani-Hamed–Thomas–Trnka’s conjectural sign-flip characterization ofAn,k,2\mathcal {A}_{n,k,2}, and Łukowski–Parisi–Spradlin–Volovich’s conjectures onm=2m=2cluster adjacencyand onpositroid tilesforAn,k,2\mathcal {A}_{n,k,2}(images of2k2k-dimensional positroid cells which map injectively intoAn,k,2\mathcal {A}_{n,k,2}). Finally, we introduce new cluster structures in the amplituhedron.
Read full abstract