Hadrons are strongly interacting particles which also participate in weak and electromagnetic interactions. They are not elementary particles and are composed of quarks, which come in six flavors; and each flavor comes in three colors (R, B, and G) as shown in Table Hadrons are specified by their quantum numbers like spin, parity, isospin, strangeness, baryon number, etc., and are classified as mesons and baryons on the basis of their quark content. Mesons are the bound states of a quark and an antiquark pair (qq), while baryons are the bound states of three quarks (qqq). They are bound in such a way that the physical states of mesons and baryons
are color singlets. The strong forces which bind the quarks together, in the case of baryons, or quarks and antiquarks together, in the case of mesons, are provided by the exchange of massless vector fields between them called the gluons. The dynamics of the strong forces, that is, the binding of the quarks and antiquarks and their interactions is described by the theory of strong interactions known as quantum chromodynamics (QCD), in a way similar to QED which describes the interactions among charged particles. In Tables 6.2 and 6.3, some of the low lying mesons and baryons, which are considered in this chapter while discussing their weak interactions, are listed along with their quark contents and other quantum numbers.
The theory of the weak interaction of hadrons is not as simple as the weak interaction of leptons, which has been described in the previous chapter. We have seen that the interaction Hamiltonian of the weak interaction, in the case of leptons, is described by
The currents here are of the V − A type. These currents lμ are constructed from the lepton fields ψl (x) described by the Dirac equation for a point spin particle. In the case of hadrons, the interaction is also of the current × current type with V − A currents at the level of quarks which are considered as point particles. However, the hadrons are not point particles.