Structure of near-threshold quasi-bound states

TL;DR

This paper investigates the internal structure of near-threshold quasi-bound states using effective field theory, revealing their compositeness and applying the relations to specific hadronic states like Lambda(1405) and a_0(980).

Contribution

It extends the weak-binding relation to unstable states with decay modes and interprets complex compositeness values, providing a model-independent analysis of hadronic structures.

Findings

Lambda(1405) is predominantly a Kbar N molecular state.

a_0(980) mainly consists of non-Kbar K components.

Generalized relations apply to unstable quasi-bound states.

Abstract

We study the compositeness of near-threshold quasi-bound 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 is utilized to generalize the weak-binding relation of the stable bound states to unstable quasi-bound 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 Lambda(1405) is dominated by the Kbar N molecular structure and that a_0(980) is dominated by the non-Kbar K component.