A cluster of results on amplituhedron tiles.

The amplituhedron is a mathematical object which was introduced to provide a geometric origin of scattering amplitudes in N = 4 super Yang-Mills theory. It generalizes cyclic polytopes and the positive Grassmannian and has a very rich combinatorics with connections to cluster algebras. In this article, we provide a series of results about tiles and tilings of the m = 4 amplituhedron. Firstly, we provide a full characterization of facets of BCFW tiles in terms of cluster variables for Gr 4 , n . Secondly, we exhibit a tiling of the m = 4 amplituhedron which involves a tile which does not come from the BCFW recurrence-the spurion tile, which also satisfies all cluster properties. Finally, strengthening the connection with cluster algebras, we show that each standard BCFW tile is the positive part of a cluster variety, which allows us to compute the canonical form of each such tile explicitly in terms of cluster variables for Gr 4 , n . This paper is a companion to our previous paper "Cluster algebras and tilings for the m = 4 amplituhedron."