A theory of the $\pi$-nucleus optical potential is developed from the exact propagator of a pion in the presence of a nucleus. The crossed $\pi$-nucleus diagrams, absent in previous work, are shown to have significant effect on the cross sections and the elastic-scattering wave functions of low-energy pions.

• Received 16 July 1973

DOI:https://doi.org/10.1103/PhysRevLett.31.610