Double poles of the<b><i>S</i></b>-matrix in a two-channel model

Abstract

The poles of the S-matrix for a two-channel model with square well potentials are calculated. It is found that the trajectories of these poles in the complex k-plane for varying coupling strength show avoided crossings. At the critical parameters the poles coincide to form a double pole. Its presence is revealed by a symmetry property of the cross section.