BCJ numerators from differential operator of multidimensional residue

Abstract

In previous works, we devised a differential operator for evaluating typical integrals appearing in the Cachazo–He–Yuan (CHY) forms and in this paper we further streamline this method. We observe that at tree level, the number of free parameters controlling the differential operator depends solely on the number of external lines, after solving the constraints arising from the scattering equations. This allows us to construct a reduction matrix that relates the parameters of a higher-order differential operator to those of a lower-order one. The reduction matrix is theory-independent and can be obtained by solving a set of explicitly given linear conditions. The repeated application of such reduction matrices eventually transforms a given tree-level CHY-like integral to a prepared form. We also provide analytic expressions for the parameters associated with any such prepared form at tree level. We finally give a compact expression for the multidimensional residue for any CHY-like integral in terms of the reduction matrices. We adopt a dual basis projector which leads to the CHY-like representation for the non-local Bern–Carrasco–Johansson (BCJ) numerators at tree level in Yang–Mills theory. These BCJ numerators are efficiently computed by the improved method involving the reduction matrix.