Off-shell extended graphic rule and the expansion of Berends-Giele currents in Yang-Mills theory

TL;DR

This paper extends the graphic rule and BCJ numerator expansion of Yang-Mills amplitudes to off-shell Berends-Giele currents, providing a new decomposition formula that maintains key symmetries and algebraic properties.

Contribution

It introduces off-shell extended numerators and a general decomposition formula for off-shell YM currents, advancing the understanding of amplitude structures beyond on-shell cases.

Findings

Off-shell numerators generated by graphs form the basis of the decomposition.

The formula reproduces on-shell YM amplitude decompositions in the on-shell limit.

Symmetric BCJ numerators satisfy Lie symmetries, relabeling, antisymmetry, and Jacobi identity.

Abstract

Tree-level color-ordered Yang-Mills (YM) amplitudes can be decomposed in terms of $(n-2)!$ bi-scalar (BS) amplitudes, whose expansion coefficients form a basis of Bern-Carrasco-Johansson (BCJ) numerators. By the help of the recursive expansion of Einstein-Yang-Mills (EYM) amplitudes, the BCJ numerators are given by polynomial functions of Lorentz contractions which are conveniently described by graphic rule. In this work, we extend the expansion of YM amplitudes to off-shell level. We define different types of off-shell extended numerators that can be generated by graphs. By the use of these extended numerators, we propose a general decomposition formula of off-shell Berends-Giele currents in YM. This formula consists of three terms: (i). an effective current which is expanded as a combination of the Berends-Giele currents in BS theory (The expansion coefficients are one type of off-shell extended numerators) (ii). a term proportional to the total momentum of on-shell lines and (iii). a term expressed by the sum of lower point Berends-Giele currents in which some polarizations and momenta are replaced by vectors proportional to off-shell momenta appropriately. In the on-shell limit, the last two terms vanish while the decomposition of effective current precisely reproduces the decomposition of on-shell YM amplitudes with the the expected coefficients (BCJ numerators in DDM basis). We further symmetrize these coefficients such that the Lie symmetries are satisfied. These symmetric BCJ numerators simultaneously satisfy the relabeling property of external lines and the algebraic properties (antisymmetry and Jacobi identity).