Empirical test for composite hadrons

We describe an empirical test to distinguish hadrons which are composites in the low-threshold channels from those which can be understood only from a more global point of view. This test is based on the existence of a pole in the K matrix in the latter case. Thus, the absence of a K pole signals a composite hadron. The test is applicable to hadrons coupled to S-wave two-body channels only. The properties of the K pole are analyzed using a field-theoretic model, and the pole and the residues are related to the wave-function renormalization constant of the hadron. We also construct coupling-constant sum rules which are connected to the test for compositeness. Applications to Λ(1405) and other hadrons are described