Compositeness of Near Threshold Quasi-Bound States

TL;DR

This paper investigates the internal structure of near-threshold unstable hadrons using effective field theory, providing a method to interpret their compositeness and applying it to specific resonances like Lambda(1405) and a_0(980).

Contribution

It extends the concept of compositeness to quasi-bound states with decay widths and offers a new interpretation method for complex compositeness values.

Findings

Lambda(1405) is mainly a Kbar N composite.

a_0(980) has a small Kbar K component.

Provides a framework for analyzing unstable hadron structures.

Abstract

We study the compositeness of unstable hadrons which lie near the two-hadron threshold. In the framework of the effective field theory, we derive the relation between the compositeness of stable bound states with observables. We then extend this relation for the quasi-bound states with finite decay width. A prescription to interpret the complex value of the compositeness is presented. With this method, we show that Lambda(1405) is dominated by the Kbar N composite component and a_0(980) have only small Kbar K fraction.