Tetraquark
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.
History
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 7000100000000000000♠100 MeV/c2.
In 2004, the DsJ(2632) state seen in Fermilab's SELEX was suggested as a possible tetraquark candidate.
In 2007, Belle announced the observation of the Z(4430) state, a ccdu tetraquark candidate. In 2014, the Large Hadron Collider experiment LHCb confirmed this resonance with a significance of over 13.9σ. There are also indications that the Y(4660), also discovered by Belle in 2007, could be a tetraquark state.