Abstract
The XYZ mesons are mesons that contain a heavy quark and antiquark but have
properties that seem to require additional constituents. Some of them are
electrically charged, so they must be tetraquark mesons whose additional
constituents are a light quark and antiquark. The list of XYZ mesons has grown
to about two dozen over the last decade. A promising approach to understanding
these mesons within QCD is the Born-Oppenheimer approximation, which reduces
the problem to the solution of the Schrodinger equation in potentials that can
be calculated using lattice QCD. The Born-Oppenheimer approximation has not yet
revealed the pattern of the XYZ mesons, but it provides a compelling framework
for understanding them from the fundamental theory.
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