Compositeness of hadrons, nuclei, and atomic systems

TL;DR

This paper reviews the concept of compositeness as a quantitative tool to understand the internal structure of hadrons, nuclei, and atomic systems, emphasizing the role of hadronic molecules and their mixing with other configurations.

Contribution

It provides a comprehensive summary of the modern understanding and application of compositeness across different quantum systems and energy scales.

Findings

Compositeness helps quantify the hadronic molecular component.

Hadronic molecules are near two-hadron thresholds but mix with other states.

The concept is applied to nuclei and atomic systems beyond hadrons.

Abstract

Recent observations of exotic hadrons have stimulated the theoretical investigation of the internal structure of hadrons. While all hadrons are ultimately composed of quarks and gluons bound by the strong interaction, quark clustering phenomena can generate hadronic molecules -- weakly bound systems of hadrons -- which are expected to emerge near two-hadron thresholds. However, it should be noted that a pure hadronic molecule is not realized, as the strong interaction induces mixing with other possible configurations. The compositeness of hadrons has been developed as a promising concept to quantitatively characterize the fraction of the hadronic molecular component. Here we summarize the modern understanding of the compositeness to study the internal structure of hadrons and review the application of the compositeness to various quantum systems in different energy scales, such as nuclei and atomic systems, in addition to hadrons.