|Standard Model of particle physics|
A tetraquark, in particle physics, is an exotic meson composed of four valence quarks. In principle, a tetraquark state may be allowed in quantum chromodynamics, the modern theory of strong interactions. Any established tetraquark state would be an example of an exotic hadron which lies outside the quark model classification.[qualify evidence]
In 2003 a particle temporarily called X(3872), by the Belle experiment in Japan, was proposed to be a tetraquark candidate, as originally theorized. The name X is a temporary name, indicating that there are still some questions about its properties to be tested. The number following is the mass of the particle in MeV/c2.
In 2007, Belle announced the observation of the Z(4430) state, a ccdu tetraquark candidate. There are also indications that the Y(4660), also discovered by Belle in 2007, could be a tetraquark state.
In 2010, two physicists from DESY and a physicist from Quaid-i-Azam University re-analyzed former experimental data and announced that, in connection with the ϒ(5S) meson (a form of bottomonium), a well-defined tetraquark resonance exists.
In February 2016, the DØ experiment announced the observation of a narrow tetraquark candidate, named X(5568), decaying to Bsπ±. However, preliminary results from LHCb, presented at the 51st Rencontres de Moriond Electroweak session, show no evidence for the state, despite a much larger sample of B0
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- V. M. Abazov; et al. (DØ collaboration) (2016). "Observation of a new B0
sπ± state". arXiv:1602.07588 [hep-ex].
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- Announcement by LHCb
- R. Aaij; et al. (LHCb collaboration) (2016). "Observation of J/ψφ structures consistent with exotic states from amplitude analysis of B+→J/ψφK+ decays". arXiv:1606.07895 [hep-ex].
- R. Aaij; et al. (LHCb collaboration) (2016). "Amplitude analysis of B+→J/ψφK+ decays". arXiv:1606.07898 [hep-ex].