Structure of near-threshold quasibound states

Abstract

We study the compositeness of near-threshold quasibound states in the framework of effective field theory. From the viewpoint of the low-energy universality, we revisit the model-independent relations between the structure of the bound state and the observables in the weak binding limit. The effective field theory enables us to generalize the weak-binding relation of the stable bound states to unstable quasibound states with decay modes. We present the interpretation of the complex values of the compositeness for the unstable states. Combining the model-independent relation and the threshold observables extracted from the experimental data, we show that $\mathrm{\ensuremath{\Lambda}}(1405)$ is dominated by the $\overline{K}N$ molecular structure and that ${a}_{0}(980)$ is dominated by the non-$\overline{K}K$ component.