Back-angle anomalies of elastic α particle scattering

An analytic continuation of the s matrix into the complex 1 plane is performed. A smooth cut-off representation for the s matrix is parametrized so that its pole nearest to the real axis dominates the elastic scattering of strongly absorbed particles. An application to α particle elastic scattering data gives information about the nature of the pole trajectory. A description of the back-angle anomalies, their energy dependence and the isotope effect is presented. (author)