The X(3872) is an exotic meson candidate with a mass of 3871.68 MeV/c2  which does not fit into the quark model because of its quantum numbers. It was first discovered in 2003 by the Belle experiment in Japan and later confirmed by several other experimental collaborations. Several theories have been proposed for its nature, such as a mesonic molecule or a diquark-antidiquark pair (tetraquark).
- Particle Data Group Live listings
- Choi, S.-K.; et al. (Belle Collaboration) (2003). "Observation of a Narrow Charmoniumlike State in Exclusive B±→K±π+π-J/ψ Decays". Physical Review Letters. 91 (26): 262001. arXiv:hep-ex/0308029. Bibcode:2003PhRvL..91z2001C. doi:10.1103/PhysRevLett.91.262001. PMID 14754041.
- Swanson, E. S. (2006). "The new heavy mesons: A status report". Physics Reports. 429 (5): 243–305. arXiv:hep-ph/0601110. Bibcode:2006PhR...429..243S. doi:10.1016/j.physrep.2006.04.003.
- Aaij, R.; et al. (LHCb collaboration) (2013). "Determination of the X(3872) meson quantum numbers". Physical Review Letters. 110 (22): 222001. arXiv:1302.6269. Bibcode:2013PhRvL.110v2001A. doi:10.1103/PhysRevLett.110.222001. PMID 23767712.
- David Harris (13 April 2008). "The charming case of X(3872)". Symmetry Magazine. Retrieved 2009-12-17.
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