Methods on compositeness and related aspects

Abstract

In many physical applications, bound states and/or resonances are observed, which raises the question whether these states are elementary or composite. Here we elaborate on several methods for calculating the compositeness $X$ of bound states and resonances in Quantum Mechanics, and in Quantum Field Theory by introducing particle number operators. For resonances $X$ is typically complex and we discuss how to get meaningful results by using certain phase transformations in the $S$ matrix.

Comments: 8 pages, 2 figures, contributed to "XVth Quark Confinement and the Hadron Spectrum Conference" (1-6 August 2022, Stavanger, Norway)