Several tetraquark candidates have been reported by particle physics experiments in the 21st century. The quark contents of these states are almost all qQ, where q represents a light quark, Q represents a heavy quark, and antiquarks are denoted with an overline. The existence and stability of tetraquark states with the qq have been discussed by theoretical physicists for a long time, however these are yet to be reported by experiments. . Confinement in Quantum Chromo Dynamics leads to the production of flux tubes connecting colour charges. The flux tubes act as attractive QCD string-like potentials. In 2003 a particle temporarily called X, 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. In 2004, the DsJ state seen in Fermilab's SELEX was suggested as a possible tetraquark candidate. In 2007, Belle announced the observation of the Z state, a tetraquark candidate. There are also indications that the Y, also discovered by Belle in 2007, could be a tetraquark state. In 2009, Fermilab announced that they have discovered a particle temporarily called Y, which may also be a tetraquark. 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 meson, a well-defined tetraquark resonance exists. In June 2013, the BES IIIexperiment in China and the Belle experiment in Japan independently reported on Zc, the first confirmed four-quark state. In 2014, the Large Hadron Collider experiment LHCb confirmed the existence of the Z state with a significance of over 13.9 σ. In February 2016, the DØ experiment reported evidence of a narrow tetraquark candidate, named X, decaying to. In December 2017, DØ also reported observing the X using a different final state. However, it was not observed in searches by the LHCb, CMS, CDF, or ATLAS experiments. In June 2016, LHCb announced the discovery of three additional tetraquark candidates, called X, X and X. In 2020, LHCb announced the discovery of a tetraquark: X.
