Analytical On-shell Calculation of Higher Order Scattering: Massive   Particles
  We demonstrate that the use of on-shell methods, involving calculation of the discontinuity across the t-channel cut associated with the exchange of a pair of massless particles, can be used to evaluate loop contributions to both the electromagnetic and gravitational scattering of massive systems. In the gravitational case, the use of factorization permits a straightforward and algebraic calculation of higher order scattering results, which were obtained previously by much more arduous Feynman diagram techniques. 
Barry R. Holstein
hep-ph gr-qc hep-th nucl-th