principal quantum number ( n = 1 , 2 , . . . ), s = s$_{1}$ + s$_{2}$ is the total spin ( s = 0 , 1), L is the relative orbital angular momentum, and J = L + S is the total angular momentum of the quark-antiquark pair. Following standard conventions, states with L = 0 , 1 , 2 are denoted as S , P , D , and so on. In addition, all hadrons can be classified according to their J PC quantum numbers, where P = (+ , − ) denotes parity and C = (+ , − ) denotes charge conjugation (which is relevant to electrically neutral states). For quarkonia, it can be shown that the parity and charge conjugation quantum numbers are related to the spin and orbital angular momentum by P = ( − ) L +1 and C = ( − ) L + $^{S}$, respectively [58]. Accordingly, heavy quarkonium states can have J PC = 0 − $^{+}$, 1 $^{−−}$, 1 + $^{−}$, 0 $^{++}$, 1$^{++}$, 2$^{++}$, for example. However, it is impossible for a heavy quarkonium state to have the quantum numbers J PC = 0 $^{−−}$, 0 + $^{−}$, 1 − $^{+}$, or 2+ $^{−}$. Quantum numbers that are forbidden for heavy quarkonia are called exotic quantum numbers. It has been widely speculated that some of the heavy quarkonium-like states could be exotic hadrons (see [28, 114] for comprehensive reviews). This would explain why the XYZ states were unanticipated by potential models that consider only quark-antiquark hadrons. Experimentally, there are some simple signals for the existence of exotic hadrons. The first is that potential models predict a certain number of states for each J PC channel, and any su- pernumerary states could be exotic hadrons. A more obvious signal would be the observation of a state with exotic J PC quantum numbers, which cannot be realized by quark-antiquark bound states. To date, no hadrons with exotic quantum numbers have been definitively ob- served. Hadrons with unusual decay modes could also be exotic. For instance, states that are above open flavour thresholds in Fig. 1.3 are kinematically allowed to decay into pairs of D mesons. Hadrons that are kinematically allowed to undergo such decays but fail to do so could be exotic. An overview of the exotic interpretations of the heavy quarkonium-like states is given in Ref. [28]. The most well known exotic hadron candidate is the X (3872), whose quantum numbers have been confirmed to be J PC = 1 ++ by the LHCb collaboration [2]. Several of its decay modes involve a J/ψ , which is the lightest spin-1 charmonium state [20]. Therefore the quark content of the X (3872) must be at least ¯ cc . However, the properties of X (3872) are incompatible with a charmonium interpretation [114]. Shortly after its discovery, it was 9