QCD amplitudes with 2 initial spacelike legs via generalised BCFW recursion

Abstract

We complete the generalisation of the BCFW recursion relation to the off-shell case, allowing for the computation of tree level scattering amplitudes for full High Energy Factorisation (HEF), i.e. with both incoming partons having a non-vanishing transverse momentum. We provide explicit results for color-ordered amplitudes with two off-shell legs in massless QCD up to 4 point, continuing the program begun in two previous papers. For the 4-fermion amplitudes, which are not BCFW-recursible, we perform a diagrammatic computation, so as to offer a complete set of expressions. We explicitly show and discuss some plots of the squared $2 \rightarrow 2$ matrix elements as functions of the differences in rapidity and azimuthal angle of the final state particles.

Two references and some more detailed comments added. Matched to the version to be published in JHEP