Eigenphases and the Generalized Breit-Wigner Approximation

Abstract

The relation between the repulsion of $S$-matrix eigenphases and "crossing" branch points (complex energies at which two or more eigenphases are equal) is examined in some detail. It is exploited to obtain a unitary one-pole approximation to the $S$ matrix for the description of an isolated resonance superimposed on a nonelastic background.