Elastic Scattering of Low Energy Pions by Nuclei and the In-Medium Isovector N Amplitude

Measurements of elastic scattering of 21.5 MeV +/- by Si, Ca, Ni and Zr were made using a single arm magnetic spectrometer and absolute calibration was made by parallel measurements of Coulomb scattering of muons. Optical model fits to the data reveal the `anomalous's-wave repulsion known from pionic atoms. Introducing a chiral-motivated density dependence of the isovector scattering amplitude, and considering the energy dependence of the free N interaction greatly improve fits to the data and remove the `anomaly'.