The role of the nucleon recoil in low--energy meson--nucleus reactions

The role of the nucleon recoil corrections in low--energy meson--nucleus interactions is examined. We demonstrate explicitly when calculations within the static approximation are justified and when the recoil terms need to be kept explictly in the propagators, depending on whether the S-wave two--nucleon intermediate state is Pauli blocked or not, while the meson is in flight. While the effect is studied in detail for $\pi d$ scattering only, a large class of other reactions is discussed for which the findings are relevant as well.