Compositeness of hadrons, nuclei, and atomic systems

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.