XYZ particle

XYZ particles 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 meson. The term arises from the names given to some of the first such particles discovered: X(3872), Y(4260) and Zc(3900), although the symbols X and Y have since been deprecated by the Particle Data Group.[2]

Theoretical significance

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.

Types of particle

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 Zc(3900) state was discovered at BESIII in 2013.

See also

References
  1. "BESIII and the XYZ mystery". CERN Courier. 30 April 2014.
  2. http://pdg.lbl.gov/2018/mobile/reviews/pdf/rpp2018-rev-naming-scheme-hadrons-m.pdf
  3. Shen Cheng-Ping and Belle collaboration (2010). "XYZ particles at Belle". Chinese Physics C. 34 (6): 615–620. arXiv:0912.2386. Bibcode:2010ChPhC..34..615S. doi:10.1088/1674-1137/34/6/001. S2CID 119181870.
  4. "New Vector Particles Observed at BESIII". Institute of High Energy Physics. 10 March 2017.
  5. Pakhlova, Galina V.; Pakhlov, Pavel N.; Eidel'man, Semen I. (2010). "Exotic charmonium". Physics-Uspekhi. 53 (3): 219–241. doi:10.3367/ufne.0180.201003a.0225.
  6. Nielsen, M. (2010). "New exotic charmonium states". Chinese Physics C. 34 (9): 1157–1162. Bibcode:2010ChPhC..34.1157N. doi:10.1088/1674-1137/34/9/002.
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