Surface Kinematics and "The" Yang-Mills Integrand

Abstract

It has been a long-standing challenge to define a canonical loop integrand for non-supersymmetric gluon scattering amplitudes in the planar limit. Naive integrands are inflicted with $1/0$ ambiguities associated with tadpoles and massless external bubbles, which destroy integrand-level gauge invariance as well as consistent on-shell factorization on single loop-cuts. In this letter, we show that this essentially kinematical obstruction to defining "the" integrand for Yang-Mills theory has a structural solution, handed to us by the formulation of gluon amplitudes in terms of curves on surfaces. This defines "surface kinematics" generalizing momenta, making it possible to define "the" integrand satisfying both a (surface generalized) notion of gauge-invariance and consistent loop-cuts. The integrand also vanishes at infinity in appropriate directions, allowing it to be recursively computed for non-supersymmetric Yang-Mills theory in any number of dimensions. We illustrate these ideas through one loop for all multiplicity, and for the simplest two-loop integrand.

Figures (4)

• Figure 1: Determining momentum of curves via homology, at tree-level (left) and one-loop (right).
• Figure 2: Tadpole (left) and external bubble (right) triangulations of the 4-point gluon one-loop surface, where each curve is assigned a different variable $X$, even when they are the same homology/have the same momentum. The scaffolding curves are dashed while the gluon internal propagators are solid lines.
• Figure 3: Different cuts of puncture disk, with curves on the original decomposing into curves on the cut surface
• Figure 4: (Left) Base triangulation for the one-point two-loop integrand. In red we have the curves with integer winding, in blue those with half-integer winding and in green those that don't have winding. (Right) Examples of different curves and their respective windings.