Structure of near-threshold s-wave resonances.

We study the structure of two-body s-wave bound states as well as resonances in the threshold energy region. We focus on the single-channel scattering where the scattering length and the effective range are given by real numbers. It is shown that, in the energy region where the effective range expansion is valid, the properties of resonances are constrained only by the position of the pole. We find that the compositeness defined through the analytic continuation of the field renormalization constant is purely imaginary and normalized for resonances. We discuss the interpretation of this quantity by examining the structure of the hadron resonance Λc(2595) in the πΣc scattering. We show that the Λc(2595) resonance requires an unnaturally large effective range and hence it is not likely a πΣc molecule.