|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]
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 have not been yet reported by experiments.
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
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
(5S) meson (a form of bottomonium), a well-defined tetraquark resonance exists.
In February 2016, the DØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying to
. In December 2017, DØ also reported observing the X(5568) using a different
s final state. However, it was not observed in searches by the LHCb, CMS, CDF, or ATLAS experiments.
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- 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.
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- Sirunyan, A. M.; et al. (CMS Collaboration) (2018). "Search for the X(5568) State Decaying into B0
sπ± 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.
- 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. doi:10.1103/PhysRevLett.120.202006. PMID 29864341.
- 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.
- Announcement by LHCb
- 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.
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