Tetraquark!
A tetraquark is a hypothetical meson (composed of one quark and one anti-quark) composed of four valence quarks. In principle, a tetraquark state may be allowed in quantum chromodynamics, the modern theory of strong interactions. However, there has been no confirmed report of a tetraquark state to date. Any established tetraquark state would be an example of an exotic hadron which lies outside the quark model classification.
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.
Further reading: http://en.wikipedia.org/wiki/Tetraquark
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