Introduction

The quark model arose from the analysis of symmetry patterns observed when particles were grouped together according to their spin and parity. When the eight mesons with Jp = 0- are displayed in a strangeness (S) versus isospin (I3) plane, they form the octet of Fig. 3.1. An identical pattern emerges for the eight vector mesons with Jp = 1- also shown in Fig. 3.1. The vector mesons are excited states of the particles in the Jp = 0- octet. The symmetry pattern was interpreted as a generalization of the isospin group SU(2) to the group SU(3) which incorporates both isospin and strangeness. Gell-Mann and Neeman (1964) proposed that the eight baryons with Jp = ½+ also belong to an octet of SU(3), thus establishing a parallelism between meson and baryon states. Finally, many static properties of the particles exhibit the SU(3) symmetry.

Since the fundamental representation of the group SU(3) is a triplet, it is natural to try to interpret the hadronic states in the octets as bound states of triplets or of triplets with antitriplets. If the fundamental fields also carry baryon number, the product of triplet ⊗ antitriplet would be mesons with zero baryon number. The product of three triplets carries baryon number and contain octets and a decuplet as was required by the observed states of baryons. This is the quark model of Gell-Mann (1964) and Zweig (1964).