Fitting ultracold resonances without a fit

Abstract

We present a numerical procedure allowing one to extract Feshbach resonance parameters from numerical calculations without relying on approximate fitting procedures. Our approach is based on a simple decomposition of the reactance matrix in terms of poles and residual background contribution, and can be applied to the general situation of inelastic overlapping resonances. A simple lineshape for overlapping inelastic resonances, equivalent to known results in the particular cases of isolated and overlapping elastic features, is also rigorously derived.