Deep Into the Amplituhedron: Amplitude Singularities at All Loops and Legs

In this letter we compute a canonical set of cuts of the integrand for MHV amplitudes in planar N=4 SYM, where all internal propagators are put on-shell. These ''deepest cuts''probe the most complicated Feynman diagrams and on-shell processes that can possibly contribute to the amplitude, but are also naturally associated with remarkably simple geometric facets of the amplituhedron. The recent reformulation of the amplituhedron in terms of combinatorial geometry directly in the kinematic (momentum-twistor) space plays a crucial role in understanding this geometry and determining the cut. This provides us with the first non-trivial results on scattering amplitudes in the theory valid for arbitrarily many loops and external particle multiplicities.