# Compositeness of hadron resonances in finite volume

## TL;DR

This paper introduces a theoretical method to analyze the internal structure of unstable hadron resonances using finite volume systems, revealing the dominant components of specific mesons and baryons.

## Contribution

A novel framework to quantify the compositeness of hadron resonances in finite volume, applicable to various mesons and baryons, and interpreting energy levels as multiple eigenstates.

## Key findings

- Kbar K component dominates about half of f_0(980) and a_0(980) wave functions.
- Lambda(1405) has 58% KbarK N component, including contributions from two eigenstates.
- Finite volume energy levels correspond to multiple infinite volume eigenstates.

## Abstract

We develop a theoretical framework to quantify the structure of unstable hadron resonances. With the help of the corresponding system in a finite volume, we define the compositeness of resonance states which can be interpreted as a probability. This framework is used to study the structure of the scalar mesons f_0(980) and a_0(980). In both mesons, the Kbar K component dominates about a half of the wave function. The method is also applied to the Lambda(1405) resonance. We argue that a single energy level in finite volume represents the two eigenstates in infinite volume. The KbarK N component of Lambda(1405), including contributions from both eigenstates, is found to be 58%, and the rest is composed of the pi Sigma and other channels.