Relevance of complex branch points for partial wave analysis

A central issue in hadron spectroscopy is to deduce--and interpret--resonance parameters, namely, pole positions and residues, from experimental data, for those are the quantities to be compared to lattice QCD or model calculations. However, not every structure in the observables derives from a resonance pole: the origin might as well be branch points, either located on the real axis (when a new channel composed of stable particles opens) or in the complex plane (when at least one of the intermediate particles is unstable). In this paper we demonstrate first the existence of such branch points in the complex plane and then show on the example of the πN P11 partial wave that it is not possible to distinguish the structures induced by the latter from a true pole signal based on elastic data alone.