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corresponding antimatter counterparts, antiquarks. The quark model suggests that there are only two kinds of hadrons: baryons that contain three quarks ( qqq ), and mesons that contain a quark and an antiquark ( q ¯ q ). Baryons and mesons naturally arrange themselves into multiplets containing hadrons with similar properties and masses that are roughly de- generate. This approximate flavour symmetry is the origin of the multiplets. The vector and pseudoscalar meson nonets are shown in Fig. 1.2. 

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In many ways, the quark model is analogous to the periodic table. Initially the periodic table served to classify the elements based on their physical and chemical properties, without 

