Remarks on unitarization methods

Abstract

It is shown that the unitarized P-wave Nambu-Goldstone scattering amplitude, constructed from the one-loop chiral perturbation theory by the K-matrix method, does not have a resonant behavior, but has instead one nearby pair of complex-conjugate poles on the physical sheet and should be rejected. This result is in constrast with other unitarization schemes such as the Pad\'e, the inverse amplitude, and the N/D methods, which produce a P-wave resonance and have the correct analytic property.