Scattering equations and BCJ relations for gauge and gravitational amplitudes with massive scalar particles
Abstract: We generalize the scattering equations to include both massless and massive particles. We construct an expression for the tree-level n-point amplitude with n − 2 gluons or gravitons and a pair of massive scalars in arbitrary spacetime dimension as a sum over the (n − 3)! solutions of the scattering equations, à la Cachazo, He, and Yuan. We derive the BCJ relations obeyed by these massive amplitudes.
Naculich, Stephen G., "Scattering equations and BCJ relations for gauge and gravitational amplitudes with massive scalar particles" (2014). Physics Faculty Publications. 110.