The X(3872) is an anomalous particle 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).
- David Harris (13 April 2008). "The charming case of X(3872)". Symmetry Magazine. Retrieved 2009-12-17.
- Particle Data Group Live listings
- Choi, S. -K.; Olsen, S.; Abe, K.; Abe, T.; Adachi, I.; Ahn, B.; Aihara, H.; Akai, K.; Akatsu, M.; Akemoto, M.; Asano, Y.; Aso, T.; Aulchenko, V.; Aushev, T.; Bakich, A.; Ban, Y.; Banerjee, S.; Bondar, A.; Bozek, A.; Bračko, M.; Brodzicka, J.; Browder, T.; Chang, P.; Chao, Y.; Chen, K. -F.; Cheon, B.; Chistov, R.; Choi, Y.; Choi, Y.; Danilov, M. (2003). "Observation of a Narrow Charmoniumlike State in Exclusive B±→K±π+π-J/ψ Decays". Physical Review Letters 91 (26). doi:10.1103/PhysRevLett.91.262001.
- Eric S. Swanson, The new heavy mesons: A status report, Physics Reports, Volume 429, Issue 5, July 2006, Pages 243–305
- LHCb collaboration: Determination of the X(3872) meson quantum numbers
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