A tetraquark state has long been suspected to be allowed by quantum chromodynamics,[1] the modern theory of strong interactions.
A tetraquark state is an example of an exotic hadron that lies outside the conventional quark model classification.
[4] In 2003, a particle temporarily called X(3872), by the Belle experiment in Japan, was proposed to be a tetraquark candidate,[6] as originally theorized.
[13] In 2014, the Large Hadron Collider experiment LHCb confirmed the existence of the Z(4430) state with a significance of over 13.9 σ.
[14][15] In February 2016, the DØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying to B0sπ±.