Ref. [72] for a review). One such possibility is a hadrons with four quarks ( qq ¯ q q ). Four- quark hadrons can be realized in two distinct ways. The first is as a weakly bound state of two colour singlet mesons [( q ¯ q ) ( q ¯ q )], which is called a molecular state. The second is as a tetraquark, which is composed of diquark clusters that have a net colour charge [( qq ) (¯ q ¯ q )] and hence is more strongly bound than a molecular state. Diquarks are best thought of as a kind of strong correlation between two quarks within a hadron [11]. Because gluons also carry colour charge, colour singlet hadrons with explicit gluonic content are also possible. Hybrids are hadrons that can be thought of as a conventional meson with an excited gluon ( qG ¯ q ). Perhaps the most exotic of all exotic hadrons are glueballs, which are composed entirely of gluons ( GG or GGG ). Note that four-quark states, hybrids and glueballs are all bosons. It should be noted that fermionic exotic hadrons are also possible, an example of which is a pentaquark ( qqqq ¯ q ). However, these will not be discussed in this thesis. The majority of the candidates for exotic hadrons exist among heavy quarkonia, all of which are bosons. A meson that is composed of two heavy quarks of the same flavour is called heavy quarko- nium. Those that are composed of charm quarks ( c ¯ c ) are called charmonia, while those that are composed of bottom quarks $^{(}$b ¯ b ) are called bottomonia. The top quark decays very rapidly via the weak interaction and does not form bound states. Because of the large masses of the charm and bottom quarks, relativistic effects are small, and hence heavy quarkonia can be approximated reasonably well using non-relativistic quantum mechanics. It is important to note that this approach does not derive directly from QCD. Rather, a potential is chosen that is inspired by QCD. The potential includes a short distance Coulombic term and a long distance term that models the effects of confinement. Spin dependent terms are crucial and relativistic corrections can also be included. The energy levels of the quarkonium system can be calculated using potential models. Each energy level, i.e. each charmonium or bottomo- nium state, is interpreted as a distinct meson. Ref. [74] provides a review of potential model methods. Potential model predictions for the low-lying members of the charmonium and bottomo- nium spectra are in excellent agreement with experiment. However, in recent years experi- 7 1.2.4 Heavy Quarkonium-like States