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Register a new userOn the Boundaries of the m=2 Amplituhedron
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Authors
Tomasz Lukowski
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Amplituhedra $mathcal{A}_{n,k}^{(m)}$ are geometric objects of great interest in modern mathematics and physics: for mathematicians they are combinatorially rich generalizations of polygons and polytopes, based on the notion of positivity; for physicists, the amplituhedron $mathcal{A}^{(4)}_{n,k}$ encodes the scattering amplitudes of the planar $mathcal{N}=4$ super Yang-Mills theory. In this paper we study the structure of boundaries for the amplituhedron $mathcal{A}_{n,k}^{(2)}$. We classify all boundaries of all dimensions and provide their graphical enumeration. We find that the boundary poset for the amplituhedron is Eulerian and show that the Euler characteristic of the amplituhedron equals one. This provides an initial step towards proving that the amplituhedron for $m=2$ is homeomorphic to a closed ball.
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Positive geometries provide a modern approach for computing scattering amplitudes in a variety of physical models. In order to facilitate the exploration of these new geometric methods, we introduce a Mathematica package called ``amplituhedronBoundaries for calculating the boundary structures of three positive geometries: the amplituhedron $mathcal{A}_{n,k}^{(m)}$, the momentum amplituhedron $mathcal{M}_{n,k}^{(m)}$ and the hypersimplex $Delta_{k,n}$. The first two geometries are relevant for scattering amplitudes in planar $mathcal{N}=4$ SYM, while the last one is a well-studied polytope appearing in many contexts in mathematics, and is closely related to $mathcal{M}_{n,k}^{(2)}$. The package includes an array of useful tools for the study of these positive geometries, including their boundary stratifications, drawing their boundary posets, and additional tools for manipulating combinatorial structures useful for positive Grassmannians.
The momentum amplituhedron is a positive geometry encoding tree-level scattering amplitudes in $mathcal{N}=4$ super Yang-Mills directly in spinor-helicity space. In this paper we classify all boundaries of the momentum amplituhedron $mathcal{M}_{n,k}$ and explain how these boundaries are related to the expected factorization channels, and soft and collinear limits of tree amplitudes. Conversely, all physical singularities of tree amplitudes are encoded in this boundary stratification. Finally, we find that the momentum amplituhedron $mathcal{M}_{n,k}$ has Euler characteristic equal to one, which provides a first step towards proving that it is homeomorphic to a ball.
In this paper we define a new object, the momentum amplituhedron, which is the long sought-after positive geometry for tree-level scattering amplitudes in $mathcal{N}=4$ super Yang-Mills theory in spinor helicity space. Inspired by the construction of the ordinary amplituhedron, we introduce bosonized spinor helicity variables to represent our external kinematical data, and restrict them to a particular positive region. The momentum amplituhedron $mathcal{M}_{n,k}$ is then the image of the positive Grassmannian via a map determined by such kinematics. The scattering amplitudes are extracted from the canonical form with logarithmic singularities on the boundaries of this geometry.
The well-known moment map maps the Grassmannian $Gr_{k+1,n}$ and the positive Grassmannian $Gr^+_{k+1,n}$ onto the hypersimplex $Delta_{k+1,n}$, which is a polytope of codimension $1$ inside $mathbb{R}^n$. Over the last decades there has been a great deal of work on matroid subdivisions (and positroid subdivisions) of the hypersimplex; these are closely connected to the tropical Grassmannian and positive tropical Grassmannian. Meanwhile any $n times (k+2)$ matrix $Z$ with maximal minors positive induces a map $tilde{Z}$ from $Gr^+_{k,n}$ to the Grassmannian $Gr_{k,k+2}$, whose image has full dimension $2k$ and is called the $m=2$ amplituhedron $A_{n,k,2}$. As the positive Grassmannian has a decomposition into positroid cells, one may ask when the images of a collection of cells of $Gr^+_{k+1,n}$ give a dissection of the hypersimplex $Delta_{k+1,n}$. By dissection, we mean that the images of these cells are disjoint and cover a dense subset of the hypersimplex, but we do not put any constraints on how their boundaries match up. Similarly, one may ask when the images of a collection of positroid cells of $Gr^+_{k,n}$ give a dissection of the amplituhedron $mathcal{A}_{n,k,2}$. In this paper we observe a remarkable connection between these two questions: in particular, one may obtain a dissection of the amplituhedron from a dissection of the hypersimplex (and vice-versa) by applying a simple operation to cells that we call the T-duality map. Moreover, if we think of points of the positive tropical Grassmannian $mbox{Trop}^+Gr_{k+1,n}$ as height functions on the hypersimplex, the corresponding positroidal subdivisions of the hypersimplex induce particularly nice dissections of the $m=2$ amplituhedron $mathcal{A}_{n,k,2}$. Along the way, we provide a new characterization of positroid polytopes and prove new results about positroidal subdivisions of the hypersimplex.
We initiate the systematic study of emph{local positive spaces} which arise in the context of the Amplituhedron construction for scattering amplitudes in planar maximally supersymmetric Yang-Mills theory. We show that all local positive spaces relevant for one-loop MHV amplitudes are characterized by certain sign-flip conditions and are associated with surprisingly simple logarithmic forms. In the maximal sign-flip case they are finite one-loop octagons. Particular combinations of sign-flip spaces can be glued into new local positive geometries. These correspond to local pentagon integrands that appear in the local expansion of the MHV one-loop amplitude. We show that, geometrically, these pentagons do emph{not} triangulate the original Amplituhedron space but rather its twin Amplituhedron-Prime. This new geometry has the same boundary structure as the Amplituhedron (and therefore the same logarithmic form) but differs in the bulk as a geometric space. On certain two-dimensional boundaries, where the Amplituhedron geometry reduces to a polygon, we check that both spaces map to the same dual polygon. Interestingly, we find that the pentagons internally triangulate that dual space. This gives a direct evidence that the chiral pentagons are natural building blocks for a yet-to-be discovered dual Amplituhedron.
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