  We derive a new integral equation that allows the calculation of the
scattering or annihilation amplitude of two particles subjected to two
potentials when the corresponding amplitude for one potential only is known. We
assume that scattering or annihilation occurs through one of the potentials,
while the other potential affects the particle wave functions. Our expression
is valid for any choice of the distorting potential and for any particle model.
Our technique does not require the expansion of the amplitude into partial
waves, and allows the study of models that are generally difficult to solve by
means of the Schroedinger equation.
