Stability of the S matrix pole

Abstract

We show how the position and residue of the S matrix pole can remain stable under changes in the form of the parametrization of the S matrix elements. We also derive a relation among the shifts in the Breit-Wigner resonance parameters under the same changes and verify the relation numerically for the case of Δ(1232). Despite its stability, the pole does not provide a unique definition of the resonance because of the existence of shadow poles. © 1990 the Indian Academy of Sciences.