Some more amplituhedra are contractible

Abstract

The amplituhedra arise as images of the totally nonnegative Grassmannians by projections that are induced by linear maps. They were introduced in Physics by Arkani-Hamed and Trnka (J High Energy Phys 10:30, 2014) as model spaces that should provide a better understanding of the scattering amplitudes of quantum field theories. The topology of the amplituhedra has been known only in a few special cases, where they turned out to be homeomorphic to balls. The amplituhedra are special cases of Grassmann polytopes introduced by Lam (in: Jerison, Kisin, Seidel, Stanley, Yau, Yau (eds) Current developments in mathematics, International Press, Somerville, 2016). In this paper we show that some further amplituhedra are homeomorphic to balls, and that some more Grassmann polytopes and amplituhedra are contractible.