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Short description: Exotic meson composed of four valence quarks

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.

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 Charm quarkCharm antiquarkDown quarkUp antiquark 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 B0sπ±.[16] In December 2017, DØ also reported observing the X(5568) using a different B0s 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 Charm quarkCharm quarkCharm antiquarkCharm antiquark 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


  1. U. Kulshreshtha; D. S. Kulshreshtha; J. P. Vary (2015). "Hamiltonian, path integral and BRST formulations of large N scalar QCD2 on the light-front and spontaneous symmetry breaking". European Physical Journal C 75 (4): 174. doi:10.1140/epjc/s10052-015-3377-x. Bibcode2015EPJC...75..174K. 
  2. 2.0 2.1 R. Aaij (2020). "Observation of structure in the J/ψ-pair mass spectrum". Science Bulletin 65 (23): 1983–1993. doi:10.1016/j.scib.2020.08.032. PMID 36659056. Bibcode2020SciBu..65.1983L. 
  3. 3.0 3.1 LHCb collaboration; Aaij, R.; Beteta, C. Abellán; Ackernley, T.; Adeva, B.; Adinolfi, M.; Afsharnia, H.; Aidala, C. A. et al. (2021-03-02). "Observation of New Resonances Decaying to J/ψK+ and J/ψϕ". Physical Review Letters 127 (8): 082001. doi:10.1103/PhysRevLett.127.082001. PMID 34477418. Bibcode2021PhRvL.127h2001A. 
  4. Si-Qiang, Luo; Kan, Chen; Xiang, Liu; Yan-Rui, Liu; Shi-Lin, Zhu (25 October 2017). "Exotic tetraquark states with the qqQQ configuration". European Physical Journal C 77:709 (10). doi:10.1140/epjc/s10052-017-5297-4. Retrieved 26 November 2017. 
  5. "The charming case of X(3872) (APS April 2008) | symmetry magazine" (in en). 2008-04-13. 
  6. D. Harris (13 April 2008). "The charming case of X(3872)". Symmetry Magazine. 
  7. L. Maiani; F. Piccinini; V. Riquer; A.D. Polosa (2005). "Diquark-antidiquarks with hidden or open charm and the nature of X(3872)". Physical Review D 71 (1): 014028. doi:10.1103/PhysRevD.71.014028. Bibcode2005PhRvD..71a4028M. 
  8. Kulshreshtha, Usha; Daya Shankar Kulshreshtha; Vary, James P. (2005). "Regge Trajectories Analysis to DSJ(2317)±, DSJ(2460)± and DSJ(2632)+ Mesons". Physical Review D 72: 017902. doi:10.1103/PhysRevD.72.017902. 
  9. G. Cotugno; R. Faccini; A.D. Polosa; C. Sabelli (2010). "Charmed Baryonium". Physical Review Letters 104 (13): 132005. doi:10.1103/PhysRevLett.104.132005. PMID 20481876. Bibcode2010PhRvL.104m2005C. 
  10. A. Minard (18 March 2009). "New Particle Throws Monkeywrench in Particle Physics". Universe Today. 
  11. Z. Matthews (27 April 2010). "Evidence grows for tetraquarks". Physics World. 
  12. 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. doi:10.1103/PhysRevLett.104.162001. PMID 20482041. Bibcode2010PhRvL.104p2001A. 
  13. E. Swanson (2013). "Viewpoint: New Particle Hints at Four-Quark Matter". Physics 6: 69. doi:10.1103/Physics.6.69. Bibcode2013PhyOJ...6...69S. 
  14. C. O'Luanaigh (9 Apr 2014). "LHCb confirms existence of exotic hadrons". CERN. 
  15. R. Aaij (2014). "Observation of the resonant character of the Z(4430) state". Physical Review Letters 112 (22): 222002. doi:10.1103/PhysRevLett.112.222002. PMID 24949760. Bibcode2014PhRvL.112v2002A. 
  16. V. M. Abazov (2016). "Observation of a new B0sπ± state". Physical Review Letters 117 (2): 022003. doi:10.1103/PhysRevLett.117.022003. PMID 27447502. Bibcode2016PhRvL.117b2003A. 
  17. Abazov, V.M. (2018). "Study of the X±(5568) state with semileptonic decays of the B0s meson". Physical Review D 97 (9): 092004. doi:10.1103/PhysRevD.97.092004. Bibcode2018PhRvD..97i2004A. 
  18. "Recent hot results & semileptonic b hadron decay". CERN. 13 March 2016. 
  19. Sirunyan, A. M. (2018). "Search for the X(5568) State Decaying into B0sπ± in Proton-Proton Collisions at √s =8  TeV". Physical Review Letters 120 (20): 202005. doi:10.1103/PhysRevLett.120.202005. PMID 29864318. 
  20. Aaltonen, T. (2018). "A search for the exotic meson X(5568) with the Collider Detector at Fermilab". Physical Review Letters 120 (20): 202006. doi:10.1103/PhysRevLett.120.202006. PMID 29864341. Bibcode2018PhRvL.120t2006A. 
  21. Aaboud, M. (2018). "Search for a Structure in the B0s π± Invariant Mass Spectrum with the ATLAS Experiment". Physical Review Letters 120 (20): 202007. doi:10.1103/PhysRevLett.120.202007. PMID 29864314. Bibcode2018PhRvL.120t2007A. 
  22. Announcement by LHCb
  23. R. Aaij (2017). "Observation of J/ψφ structures consistent with exotic states from amplitude analysis of B+→J/ψφK+ decays". Physical Review Letters 118 (2): 022003. doi:10.1103/PhysRevLett.118.022003. PMID 28128595. Bibcode2017PhRvL.118b2003A. 
  24. R. Aaij (2017). "Amplitude analysis of B+→J/ψφK+ decays". Physical Review D 95 (1): 012002. doi:10.1103/PhysRevD.95.012002. Bibcode2017PhRvD..95a2002A. 
  25. "Observation of a four-charm-quark tetraquark.". LHCb - Large Hadron Collider beauty experiment. CERN. 1 July 2020. 
  26. "ATLAS observes potential four-charm tetraquark" (in en). 
  27. "LHCb discovers three new exotic particles". CERN. 5 July 2022. Retrieved 8 July 2022. 

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