广义散射振幅中的平面性：PK多面体、广义根系和Worldsheet缔合面
Planarity in Generalized Scattering Amplitudes: PK Polytope, Generalized Root Systems and Worldsheet Associahedra
摘要：
在本文中，我们研究了广义散射振幅的平面性的作用，通过几个密切相互作用的结构在组合，代数和热带几何。Cachazo-Early-Guevara-Mizera（CEGM）最近提出的广义双伴随标量振幅是关于$\binom{n}{k}-n$自变量的齐次有理函数$-(k-1)(n-k-1)$，它的极点可以直接由正热带格拉斯曼射线构造。我们介绍了每对整数$(k,n)$与$2\le k\le n-2$一个系统的广义正根出现作为一个专业化的平面基础的运动不变量。证明了高根多胞形$\mathcal{R}^{(k)}_{n-k}$的体积为k维Catalan数$C^{(k)}_{n-k}$，通过旗幺模三角剖分成单形，在双射中与$k$ -元子集的非交叉集合.我们还给出了一个双射之间的某些positroidal细分，称为三脚架，超单纯形$Δ_{3,n}$和非交叉对的3元素子集，不弱分离。我们证明了最近由Cachazo和作者引入的平面动力学（PK）多面体的刻面正是$\binom{n}{k}-n$广义正根。我们发现，PK专业化的广义双伴随振幅评估$C^{(k)}_{n-k}$。展望未来，我们给出了定义方程和猜想显式解使用$(\mathbb{CP}^{n-k-1})^{\times (k-1)}$通过一个概念的兼容度的非交叉集合，为两个参数的家庭的广义世界表缔合面$\mathcal{W}^+_{k,n}$。当$k=2$特殊化为$\mathbb{CP}^{1}$中$n$不同点的构形空间$M_{0,n}$的某个二面角不变的部分紧化时。许多详细的例子，以激励未来的工作。
英文摘要：
In this paper we study the role of planarity in generalized scattering amplitudes, through several closely interacting structures in combinatorics, algebraic and tropical geometry. The generalized biadjoint scalar amplitude, introduced recently by Cachazo-Early-Guevara-Mizera (CEGM), is a rational function of homogeneous degree $-(k-1)(n-k-1)$ in $\binom{n}{k}-n$ independent variables; its poles can be constructed directly from the rays of the positive tropical Grassmannian. We introduce for each pair of integers $(k,n)$ with $2\le k\le n-2$ a system of generalized positive roots which arises as a specialization of the planar basis of kinematic invariants. We prove that the higher root polytope $\mathcal{R}^{(k)}_{n-k}$ has volume the k-dimensional Catalan number $C^{(k)}_{n-k}$, via a flag unimodular triangulation into simplices, in bijection with noncrossing collections of $k$-element subsets. We also give a bijection between certain positroidal subdivisions, called tripods, of the hypersimplex $Δ_{3,n}$ and noncrossing pairs of 3-element subsets that are not weakly separated. We show that the facets of the Planar Kinematics (PK) polytope, introduced recently by Cachazo and the author, are exactly the $\binom{n}{k}-n$ generalized positive roots. We show that the PK specialization of the generalized biadjoint amplitude evaluates to $C^{(k)}_{n-k}$. Looking forward, we give defining equations and conjecture explicit solutions using $(\mathbb{CP}^{n-k-1})^{\times (k-1)}$ via a notion of compatibility degree for noncrossing collections, for a two parameter family of generalized worldsheet associahedra $\mathcal{W}^+_{k,n}$. These specialize when $k=2$ to a certain dihedrally invariant partial compactification of the configuration space $M_{0,n}$ of $n$ distinct points in $\mathbb{CP}^{1}$. Many detailed examples are given throughout to motivate future work.
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