The Exotic XYZ Charmonium-Like Mesons

Charmonium, the spectroscopy of c (c) over bar mesons, has recently enjoyed a renaissance with the discovery of several missing states and numerous unexpected charmonium-like resonances. These discoveries were made possible by the extremely large data samples made available by the B factories at SLAC and KEK, as well as CESR. Conventional c (c) over bar states are well described by quark potential models; however, many of the newly discovered charmonium-like mesons do not seem to fit into the conventional c (c) over bar spectrum. There is growing evidence that at least some of these new states are exotic, e.g., new forms of hadronic matter such as mesonic molecules, tetraquarks, and/or hybrid mesons. In this review we describe expectations for the properties of conventional charmonium states and the predictions for molecules, tetraquarks, and hybrids and the various processes that produce them. We examine the evidence for the new candidate exotic mesons, possible explanations, and experimental measurements that might reveal the nature of these states.