Off-Shell Unitarity for Two Spin-½ Particles

Abstract

The implications of off-shell unitarity for the system of two spin-½ particles are examined. It is found that unitarity and time-reversal invariance imply a parametrization of the half-off-shell scattering amplitude which is identical to a formula previously found from a potential model. The amplitude is expressed in terms of the on-shell phase shifts and additional real quasi-phase-parameters.