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.
Highlights
The XYZ mesons are more than two dozen new ccand bbmesons that have been discovered since 2003 [1]
Z+(4430), discovered by the Belle Collaboration in 2007 [4]. It decays into ψ(2S ) π+, which implies that it must be a tetraquark meson with constituents ccud
Y(4140), discovered by the CDF Collaboration in 2009 [5]. It decays into J/ψ φ, which suggests that it could be a tetraquark meson with constituents ccss
Summary
The XYZ mesons are more than two dozen new ccand bbmesons that have been discovered since 2003 [1]. Z+(4430), discovered by the Belle Collaboration in 2007 [4] It decays into ψ(2S ) π+, which implies that it must be a tetraquark meson with constituents ccud. Y(4140), discovered by the CDF Collaboration in 2009 [5] It decays into J/ψ φ, which suggests that it could be a tetraquark meson with constituents ccss. ZΥb+π(+1,0w61h0ic)hanimd pZlb+ie(1s0th6a5t0t)h, edyismcouvsetrbeed by the Belle Collaboration in 2011 [6] Zc+(3900), discovered by the BESIII Collaboration in 2013 [7] It decays into J/ψ π+, which implies that it must be a tetraquark meson with constituents ccud. They present a major challenge to our understanding of the QCD spectrum!
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