ments have begun to probe the mass region that higher mass charmonium states are expected to occupy. The results of these experiments have been quite surprising: numerous states that were not predicted by potential models have been found in the 3 . 8 − 4 . 7 GeV mass region, and a few unanticipated states have been found within the bottomonium mass region as well [28]. These anomalous states are called heavy quarkonium-like, or XYZ states. The current ex- perimental situation is summarized in detail in Ref. [20]. Fig. 1.3 shows the charmonium spectrum, including many of the charmonium-like states. Charmonium states are labeled using spectroscopic notation n 2 s $^{+1}$L$_{J}$ , where n is the 8 Figure 1.3: The charmonium spectrum. Black lines denote charmonium states, and red dots indicate charmonium-like states. Blue lines indicate the thresholds at which states can decay into a pair of D mesons, which contain a charm quark and a light quark. Note that most of the J PC quantum numbers assigned to the XYZ states here are speculative (Ref. [20] lists the possible J PC for each XYZ state). Figure taken from Ref. [53].