S-matrix and R-matrix determination of the low-energy ⁵He and ⁵Li resonance parameters

We study the low-energy 3/2^- and 1/2^- states of ⁵He and ⁵Li in a microscopic cluster model. The scattering phase shifts of Bond (α+n) and of Schwandt (α+p), respectively, are well reproduced. We determine the resonance parameters by localizing the poles of the analytically continued S matrix at complex energies. Our results differ from conventional R-matrix resonance parameters, that were extracted from experimental data using the definition of a resonance based on the positions and widths of reaction cross section peaks. However, they nicely agree with the results of an extended R-matrix method that works at complex energies.