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In particle physics, a tetraquark is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, [1] the modern theory of strong interactions. A tetraquark state is an example of an exotic hadron which lies outside the conventional quark model classification. A number of different types of tetraquark have been observed. [2] [3]

History and discoveries

Several tetraquark candidates have been reported by particle physics experiments in the 21st century. The quark contents of these states are almost all qqQQ, where q represents a light ( up, down or strange) quark, Q represents a heavy ( charm or bottom) quark, and antiquarks are denoted with an overline. The existence and stability of tetraquark states with the qqQQ (or qqQQ) have been discussed by theoretical physicists for a long time, however these are yet to be reported by experiments. [4]

Colour flux tubes produced by four static quark and antiquark charges, computed in lattice QCD. [5] Confinement in quantum chromodynamics leads to the production of flux tubes connecting colour charges. The flux tubes act as attractive QCD string-like potentials.
Timeline

In 2003, a particle temporarily called X(3872), by the Belle experiment in Japan, was proposed to be a tetraquark candidate, [6] as originally theorized. [7] 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 2004, the DsJ(2632) state seen in Fermilab's SELEX was suggested as a possible tetraquark candidate. [8]

In 2007, Belle announced the observation of the Z(4430) state, a
c
c
d
u
 tetraquark candidate. There are also indications that the Y(4660), also discovered by Belle in 2007, could be a tetraquark state. [9]

In 2009, Fermilab announced that they have discovered a particle temporarily called Y(4140), which may also be a tetraquark. [10]

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. [11] [12]

In June 2013, the BES III experiment in China and the Belle experiment in Japan independently reported on Zc(3900), the first confirmed four-quark state. [13]

In 2014, the Large Hadron Collider experiment LHCb confirmed the existence of the Z(4430) state with a significance of over 13.9 σ. [14] [15]

In February 2016, the DØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying to
B0
s
π±
. [16] In December 2017, DØ also reported observing the X(5568) using a different
B0
s final state. [17] However, it was not observed in searches by the LHCb, [18] CMS, [19] CDF, [20] or ATLAS [21] experiments.

In June 2016, LHCb announced the discovery of three additional tetraquark candidates, called X(4274), X(4500) and X(4700). [22] [23] [24]

In 2020, LHCb announced the discovery of a
c
c
c
c
 tetraquark: X(6900). [2] [25] In 2022, ATLAS observed X(6900). [26]

In 2021, LHCb announced the discovery of four additional tetraquarks, including ccus. [3]

In 2022, LHCb announced the discovery of csud and csud. [27]

See also

References

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  22. ^ Announcement by LHCb
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External links

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