Spin Reversal in Scattering Processes

Abstract

The effect on the scattering matrix element of reversing the spins of all particles is discussed. An exact relation is shown to be the consequence of invariance under combined space and time inversion and the unitarity of the $S$ matrix. In lowest order perturbation theory this gives a symmetry property of the matrix element. The implication for particles in arbitrary states of polarization is pointed out.