Investigations of the pion-nucleus optical potential from pionic atoms

The explicit dependence of the zero-energy π^?-nucleus optical potential on its various parameters, as well as its implicit dependence on neutron radii, is investigated for a selected set of particularly accurate level shift and width data. Among the various effects studied are some possible variations in the form of the Lorentz-Lorenz term, terms induced by the so-called “angle-transformation”, and the smearing of the π 2N terms which allows for a finite range in the absorption process. The pionic atom data are equally well fitted by any of the several forms used for the optical potential, with the persistent failure to fit the recently reported widths of 3d levels in ¹⁸¹Ta, Re and ²⁰⁹Bi and the width of the 1s level in ²³Na.