Backward dispersion relations and pion-/sup 4/He phase-shift analyses

The usefulness of backward dispersion relations in nuclear physics is discussed. As an example, pion-/sup 4/He elastic scattering is considered. It is shown how the use of backward dispersion relations can help in reducing ambiguities of phase-shift analyses. In particular, the solution with the S-wave changing sign twice between 150 and 260 MeV and with the real part of the backward scattering amplitude becoming positive at approximately 140 MeV and exhibiting resonating behaviour has been suggested as the most probable one. (8 refs).