Several tetraquark candidates have been reported by particle physics experiments in the 21st century. The quark contents of these states are almost all qqQQ, where q represents a light (up,down orstrange) quark, Q represents a heavy (charm orbottom) quark, and antiquarks are denoted with an overline. The existence and stability of tetraquark states with the qqQQ (orqqQQ) have been discussed by theoretical physicists for a long time, however these are yet to be reported by experiments.[4]
Colour flux tubes produced by four static quark and antiquark charges, computed inlattice QCD.[5] Confinement in quantum chromodynamics leads to the production offlux tubes connecting colour charges. The flux tubes act as attractiveQCD string-like potentials.
Timeline
In 2003, a particle temporarily calledX(3872), by theBelle experiment inJapan, was proposed to be a tetraquark candidate,[6] as originally theorized.[7] 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 inMeV/c2.
In 2004, the DsJ(2632) state seen inFermilab's SELEX was suggested as a possible tetraquark candidate.[8]
In 2007, Belle announced the observation of theZ(4430) state, accdu tetraquark candidate. There are also indications that theY(4660), also discovered by Belle in 2007, could be a tetraquark state.[9]
In 2009,Fermilab announced that they have discovered a particle temporarily calledY(4140), which may also be a tetraquark.[10]
In June 2013, theBES III experiment in China and the Belle experiment in Japan independently reported onZc(3900), the first confirmed four-quark state.[13]
In February 2016, theDØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying toB0 sπ± .[16]In December 2017, DØ also reported observing the X(5568) using a differentB0 s final state.[17]However, it was not observed in searches by the LHCb,[18]CMS,[19]CDF,[20] orATLAS[21] experiments.
In June 2016, LHCb announced the discovery of three additional tetraquark candidates, called X(4274), X(4500) and X(4700).[22][23][24]
In 2020, LHCb announced the discovery of acccctetraquark: X(6900).[2][25] In 2022, ATLAS also observed X(6900),[26] and in 2023, CMS reported an observation of three such states, X(6600), X(6900), and X(7300).[27]
In 2021, LHCb announced the discovery of four additional tetraquarks, including ccus.[3]
In 2022, LHCb announced the discovery of csud and csud.[28]
