External leg corrections in the unitarity method

Abstract

Unitarity cuts diverge in the channel of a single massive external fermion. We propose an off-shell continuation of the momentum that allows a finite evaluation of the unitarity cuts. If the cut is taken with complete amplitudes on each side, our continuation and expansion around the on-shell configuration produces the finite contribution to the bubble coefficient. Finite parts in the expansion of the external leg counterterms must be included explicitly as well.

Comments: 28 pages, 9 figures. Published version. Eq. (B.17) corrected, minor clarifications, typos fixed