XYZ particle

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In particle physics, XYZ particles (also referred to as XYZ states) are recently-discovered heavy mesons whose properties do not appear to fit the standard picture of charmonium and bottomonium states.^[1] They are therefore types of exotic mesons. The term arises from the names given to some of the first such particles discovered: X(3872), Y(4260) and Z_c(3900), although the symbols X and Y have since been deprecated by the Particle Data Group.^[2]

Since 2003 a frontier for the Standard Model (SM) has emerged at low energies through XYZ particle discoveries. The well-established theory of Quantum Chromodynamics (QCD) is tested by many exotic charmonium discoveries since the X(3872) was first identified at the Belle experiment in 2003.^[3] The basic model of hadron physics is the assembling of quarks into groups of 3 (baryons) or a quark and anti-quark pair (mesons). A meson with a charm quark and an anti-charm quark is called charmonium, and the same parallels with the bottom quark and bottomonium. More than two dozen previously unpredicted charmonium- and bottomonium-like states have been discovered, and the understanding of heavy quarkonium physics is undetermined.^[4] Previously postulated exotic Standard Model states might apply to these new unique particles. One proposed state is the hybrid state of a quark, anti-quark, and a gluon, sometimes mentioned with charm quarks as an excited charmonium. A multi-quark state of 4 or more quarks (tetraquark, pentaquark, etc.) is also proposed as well as a molecule-like state of multiple mesons.^[5][6] While each of these three types of states have had some success of explaining the newly discovered particle, a complete explanation has not been found.

The first charmonium state with an unpredicted mass was X(3872). The Belle collaboration was searching for the B -> K π+ π- J/ψ decay when they discovered a peak in the π+ π- J/ψ invariant energy at 3872 with JCP quantum numbers of 1++. X(3872) was quickly confirmed by BaBar, CDF, and D0. The mass of X(3872) is close to the mass of DD* and makes it a candidate as a meson molecule or a possible tetraquark. In 2005 the BaBar collaboration found Y(4260) from Initial state radiation as well in π+ π- J/ψ production. Again a charmonium-like particle with a large coupling to final states without open charm mesons. Continued search shows a lack of an observation in the inclusive hadronic cross section. The BES III collaboration in 2012 started taking data at 4260 MeV and could observe direct production instead of B decay or Initial State Radiation to continue the study with a higher luminosity. The Z_c(3900) state was discovered at BESIII in 2013.

• Exotic meson
• Z(4430)