Introduction

Hadrons are strongly interacting particles which also participate in weak andelectromagnetic interactions. They are not elementary particles and arecomposed of quarks, which come in six flavors; and each flavor comes inthree colors (R, B, andG) as shown in Table Hadrons are specified by their quantumnumbers like spin, parity, isospin, strangeness, baryon number, etc., andare 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 threequarks (qqq). They are bound in such a way that thephysical states of mesons and baryons

are color singlets. The strong forces which bind the quarks together, in thecase of baryons, or quarks and antiquarks together, in the case of mesons,are provided by the exchange of massless vector fields between them calledthe gluons. The dynamics of the strong forces, that is, the binding of thequarks and antiquarks and their interactions is described by the theory ofstrong interactions known as quantum chromodynamics (QCD), in a way similarto QED which describes the interactions among charged particles. In Tables6.2 and 6.3, some of the low lying mesons and baryons, which are consideredin this chapter while discussing their weak interactions, are listed alongwith their quark contents and other quantum numbers.

The theory of the weak interaction of hadrons is not as simple as the weakinteraction of leptons, which has been described in the previous chapter. Wehave seen that the interaction Hamiltonian of the weak interaction, in thecase of leptons, is described by

where

The currents here are of the V − A type. Thesecurrents lμ are constructed from the lepton fieldsψl (x) described by the Diracequation for a point spin particle. In the case of hadrons, the interactionis also of the current × current type with V −A currents at the level of quarks which are considered as pointparticles. However, the hadrons are not point particles.