Interactions of Hadrons at Low Energies and the Static Quark Model

Abstract

The list of “elementary particles” is much longer and not as well defined as the “elementary constituents” (quarks and leptons). In this chapter, some simple problems dealing with the structure and classification of particles made of quarks and interacting via the strong interaction are presented. As soon as the 1960s, the growing number of hadrons and the repetition of some characteristics led some to believe that hadrons are not “elementary particles,” but that they are made up of smaller objects, the quarks. Initially (as also considered here) it was thought that only three quark types (u,d,s) plus their antiquarks existed. Today, six quark flavors u, d, s, c, b, t, are known. Supplement 7.1 presents the ingredients used to sum both the angular momentum and the isotopic spin of particles, namely the Clebsch-Gordan coefficients. We also present how the isotopic spin helps to classify the hadrons made of u, d quarks into multiplets.