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Abstract

Following the direction of 1712.09990 and 1712.09994, this article continues to excavate more interesting aspects of the 4-particle amplituhedron for a better understanding of the 4-particle integrand of planar N=4 SYM to all loop orders, from the perspective of positive geometry. At 3-loop order, we introduce a much more refined dissection of the amplituhedron to understand its essential structure and maximally simplify its direct calculation, by fully utilizing its symmetry as well as the efficient Mondrian way for reorganizing all contributing pieces. Although significantly improved, this approach immediately encounters its technical bottleneck at 4-loop. Still, we manage to alleviate this difficulty by imitating the traditional (generalized) unitarity cuts, which is to use the so-called positive cuts. Given a basis of dual conformally invariant (DCI) loop integrals, we can figure out the coefficient of each DCI topology using its dlog form via positivity conditions. Explicit examples include all 2+5 non-rung-rule topologies at 4- and 5-loop respectively. These results remarkably agree with previous knowledge, which confirms the validity of amplituhedron up to 5-loop and develops a new approach of determining the coefficient of each distinct DCI loop integral.