Theoretical Interpretations of the XYZ States

The $XYZ$ mesons are unexpected $c \bar c$ and $b \bar b$ mesons that have been discovered during the last decade. The models for the $XYZ$ mesons that have been proposed, none of which have revealed a compelling pattern, include conventional quarkonium, quarkonium hybrids, and quarkonium tetraquarks (whose four constituents can be clustered in various ways). The Born-Oppenheimer approximation provides a coherent QCD framework for describing the $XYZ$ mesons that can be informed by lattice QCD. The additional hints that will be provided by current and upcoming experiments guarantee the eventual solution of the $XYZ$ puzzle.