# XYZ particle

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/XYZ_particle
> Markdown URL: https://mediated.wiki/source/XYZ_particle.md
> Source: https://en.wikipedia.org/wiki/XYZ_particle
> Source revision: 1302924874
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

In [particle physics](/source/Particle_physics), **XYZ particles** (also referred to as **XYZ states**) are recently discovered heavy [mesons](/source/Meson) whose properties do not appear to fit the standard picture of [charmonium](/source/Charmonium) and [bottomonium](/source/Bottomonium) states.[1] They are therefore types of [exotic mesons](/source/Exotic_meson). The term arises from the names given to some of the first such particles discovered: [X(3872)](/source/X(3872)), [Y(4260)](/source/Y(4260)) and [Zc(3900)](/source/Zc(3900)), although the symbols X and Y have since been deprecated by the [Particle Data Group](/source/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](/source/Quantum_Chromodynamics) (QCD) is tested by many exotic charmonium discoveries since the X(3872) was first identified at the [Belle experiment](/source/Belle_experiment) in 2003.[3] The basic model of hadron physics is the assembling of [quarks](/source/Quarks) into groups of 3 ([baryons](/source/Baryons)) or a quark and anti-quark pair ([mesons](/source/Mesons)). A meson with a [charm quark](/source/Charm_quark) and an [anti-charm quark](/source/Antiquark) is called *charmonium*, and the same parallels with the [bottom quark](/source/Bottom_quark) and [bottomonium](/source/Bottomonium). More than two dozen previously unpredicted charmonium- and bottomonium-like states have been discovered, and the understanding of heavy [quarkonium](/source/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](/source/Gluon), sometimes mentioned with charm quarks as an excited charmonium. A multi-quark state of 4 or more quarks ([tetraquark](/source/Tetraquark), [pentaquark](/source/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)](/source/X(3872)). The [Belle](/source/Belle_experiment) collaboration was searching for the [B -> K π+ π- J/ψ](/source/B_meson) decay when they discovered a peak in the π+ π- J/ψ invariant energy at 3872 with JCP [quantum numbers](/source/Quantum_numbers) of 1++. X(3872) was quickly confirmed by [BaBar](/source/BaBar), [CDF](/source/Collider_Detector_at_Fermilab), and [D0](/source/D%C3%98_experiment). The mass of X(3872) is close to the mass of [DD*](/source/D_meson) and makes it a candidate as a meson molecule or a possible tetraquark. In 2005 the [BaBar](/source/BaBar) collaboration found Y(4260) from [Initial state radiation](/source/Initial_state_radiation) as well in [π+ π](/source/%CE%A0_meson)- [J/ψ](/source/J/%CF%88) 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](/source/BES_III) collaboration in 2012 started taking data at 4260 [MeV](/source/MeV) and could observe direct production instead of [B decay](/source/B_meson) or [Initial State Radiation](/source/Initial_and_final_state_radiation) to continue the study with a higher luminosity. The Zc(3900) state was discovered at BESIII in 2013.

## See also

- [Exotic meson](/source/Exotic_meson)
- [Z(4430)](/source/Z(4430))

## References

1. ["BESIII and the XYZ mystery"](https://web.archive.org/web/20170415015950/http://cerncourier.com/cws/article/cern/56943). *CERN Courier*. 30 April 2014. Archived from [the original](http://cerncourier.com/cws/article/cern/56943) on 15 April 2017. Retrieved 14 April 2017.

1. ["Naming scheme for hadrons"](https://pdg.lbl.gov/2018/mobile/reviews/pdf/rpp2018-rev-naming-scheme-hadrons-m.pdf). *[Particle Data Group](/source/Particle_Data_Group)*

1. Shen Cheng-Ping and Belle collaboration (2010). "XYZ particles at Belle". *Chinese Physics C*. **34** (6): 615–620. [arXiv:0912.2386](https://arxiv.org/abs/0912.2386). [Bibcode:2010ChPhC..34..615S](https://ui.adsabs.harvard.edu/abs/2010ChPhC..34..615S). [doi:10.1088/1674-1137/34/6/001](https://doi.org/10.1088/1674-1137/34/6/001). [S2CID 119181870](https://api.semanticscholar.org/CorpusID:119181870)

1. ["New Vector Particles Observed at BESIII"](https://web.archive.org/web/20170414162137/http://english.ihep.cas.cn/doc/1993.html). *Institute of High Energy Physics*. 10 March 2017. Archived from [the original](http://english.ihep.cas.cn/doc/1993.html) on 14 April 2017. Retrieved 14 April 2017.

1. 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](https://doi.org/10.3367/ufne.0180.201003a.0225). [S2CID 250860230](https://api.semanticscholar.org/CorpusID:250860230)

1. Nielsen, M. (2010). "New exotic charmonium states". *Chinese Physics C*. **34** (9): 1157–1162. [Bibcode:2010ChPhC..34.1157N](https://ui.adsabs.harvard.edu/abs/2010ChPhC..34.1157N). [doi:10.1088/1674-1137/34/9/002](https://doi.org/10.1088/1674-1137/34/9/002). [S2CID 250763143](https://api.semanticscholar.org/CorpusID:250763143)

---
Adapted from the Wikipedia article [XYZ particle](https://en.wikipedia.org/wiki/XYZ_particle) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/XYZ_particle?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.