Tetraquark

A tetraquark, in particle physics, 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.

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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.[2]

Colour flux tubes produced by four static quark and antiquark charges, computed in lattice QCD.[3] Confinement in Quantum Chromo Dynamics leads to the production of flux tubes connecting colour charges. The flux tubes act as attractive QCD string-like potentials.

In 2003 a particle temporarily called X(3872), by the Belle experiment in Japan, was proposed to be a tetraquark candidate,[4] as originally theorized.[5] 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.[6]

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.[7]

In 2009, Fermilab announced that they have discovered a particle temporarily called Y(4140), which may also be a tetraquark.[8]

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.[9][10]

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.[11]

In 2014, the Large Hadron Collider experiment LHCb confirmed the existence of the Z(4430) state with a significance of over 13.9 σ.[12][13]

In February 2016, the DØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying to
B0
s
π±
.[14] In December 2017, DØ also reported observing the X(5568) using a different
B0
s final state.[15] However, it was not observed in searches by the LHCb,[16] CMS,[17] CDF,[18] or ATLAS[19] experiments.

In June 2016, LHCb announced the discovery of three additional tetraquark candidates, called X(4274), X(4500) and X(4700).[20][21][22]

In 2020, LHCb announced the discovery of a
c

c

c

c
 tetraquark: X(6900).[23][24]

See also

References
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  2. Si-Qiang, Luo; Kan, Chen; Xiang, Liu; Yan-Rui, Liu; Shi-Lin, Zhu (25 October 2017). "Exotic tetraquark states with the qqQQ configuration" (PDF). European Physical Journal C. 77:709 (10). doi:10.1140/epjc/s10052-017-5297-4. S2CID 119377466. Retrieved 26 November 2017.
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  9. Z. Matthews (27 April 2010). "Evidence grows for tetraquarks". Physics World. Retrieved 2014-04-12.
  10. A. Ali; C. Hambrock; M.J. Aslam (2010). "Tetraquark Interpretation of the BELLE Data on the Anomalous Υ(1S)π+π− and Υ(2S)π+π− Production near the Υ(5S) Resonance". Physical Review Letters. 104 (16): 162001. arXiv:0912.5016. Bibcode:2010PhRvL.104p2001A. doi:10.1103/PhysRevLett.104.162001. PMID 20482041.
  11. E. Swanson (2013). "Viewpoint: New Particle Hints at Four-Quark Matter". Physics. 6: 69. Bibcode:2013PhyOJ...6...69S. doi:10.1103/Physics.6.69.
  12. C. O'Luanaigh (9 Apr 2014). "LHCb confirms existence of exotic hadrons". CERN. Retrieved 2016-04-04.
  13. R. Aaij; et al. (LHCb collaboration) (2014). "Observation of the resonant character of the Z(4430)− state". Physical Review Letters. 112 (22): 222002. arXiv:1404.1903. Bibcode:2014PhRvL.112v2002A. doi:10.1103/PhysRevLett.112.222002. PMID 24949760. S2CID 904429.
  14. V. M. Abazov; et al. (D0 collaboration) (2016). "Observation of a new
    B0
    s
    π±
     state". Physical Review Letters. 117 (2): 022003. arXiv:1602.07588. Bibcode:2016PhRvL.117b2003A. doi:10.1103/PhysRevLett.117.022003. PMID 27447502. S2CID 7789961.
  15. Abazov, V.M.; et al. (D0 collaboration) (2018). "Study of the X±(5568) state with semileptonic decays of the B0
    s     meson". Physical Review D. 97 (9): 092004. arXiv:1712.10176. doi:10.1103/PhysRevD.97.092004.
  16. J. van Tilburg (13 March 2016). "Recent hot results & semileptonic b hadron decay" (PDF). CERN. Retrieved 2016-04-04.
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    s    π± in Proton-Proton Collisions at √s =8  TeV". Physical Review Letters. 120 (20): 202005. arXiv:1712.06144. doi:10.1103/PhysRevLett.120.202005. PMID 29864318.
  18. Aaltonen, T.; et al. (CDF Collaboration) (2018). "A search for the exotic meson X(5568) with the Collider Detector at Fermilab". Physical Review Letters. 120 (20): 202006. arXiv:1712.09620. Bibcode:2018PhRvL.120t2006A. doi:10.1103/PhysRevLett.120.202006. PMID 29864341. S2CID 43934060.
  19. Aaboud, M.; et al. (ATLAS Collaboration) (2018). "Search for a Structure in the B0
    s     π± Invariant Mass Spectrum with the ATLAS Experiment". Physical Review Letters. 120 (20): 202007. arXiv:1802.01840. doi:10.1103/PhysRevLett.120.202007. PMID 29864314.
  20. Announcement by LHCb
  21. R. Aaij; et al. (LHCb collaboration) (2017). "Observation of J/ψφ structures consistent with exotic states from amplitude analysis of B+→J/ψφK+ decays". Physical Review Letters. 118 (2): 022003. arXiv:1606.07895. Bibcode:2017PhRvL.118b2003A. doi:10.1103/PhysRevLett.118.022003. PMID 28128595. S2CID 206284149.
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