Table 1.3: Phenomenological values of the light and heavy quark masses as given in Ref. [20]. Flavour Mass (MeV) Flavour Mass (GeV) u 2.3 c 1.28 d 4.8 b 4.18 s 95 t 173.07 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. 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 Figure 1.2: The vector (left) and pseudoscalar (right) meson nonets. The labels Y and I$_{3}$ indicate the hypercharge and isospin quantum numbers, respectively. The masses of all of these particles are less than about 1 GeV, and typically vary by a few hundred MeV within each nonet. However, the pions ( π − , π $^{0}$, π$^{+}$) are anomalously light. The reason for this emerges naturally from QCD. The figures are from Ref. [105]. I$_{3}$ Y • ρ − • ρ 0 • ρ + ω φ K ∗− ¯ ∗ 0 • K ∗ 0 • K ∗ + 5