Angular Momentum Analysis of Spin 1/2 x 1/2 Scattering including Singlet-Triplet Mixing

The angular momentum (partial wave) reduction of the Lippmann--Schwinger equation describing the interaction of two spin 1/2 particles is extended to the case in which the spin singlet and triplet states are coupled. A straight forward method for obtaining the angular momentum decomposition of the general potential is indicated. The derived formalism is needed to describe the interaction between two nonidentical spin--1/2 particles or between two nucleons when isospin symmetry is violated. The resulting modification of the Stapp phase--shift analysis is given.