Low-energy-pion elastic scattering from light nuclei

Simple $\pi$-nucleus optical potentials constructed from $\pi -N$ phase shifts yield poor fits to elastic scattering from He and C below 75 MeV. Forcing a fit gives potentials which are pion producing or in violation of unitarity. The forward position (∼65°) and breadth of the minimum are shown to be partially responsible. These features are also shown to require a much stronger $s$-wave interaction with nuclei than that derived from $\pi -N$ scattering.