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 or
strange) quark, Q represents a heavy (
charm or
bottom) quark, and antiquarks are denoted with an overline. The existence and stability of tetraquark states with the qqQQ (or qqQQ) 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 in
lattice QCD.[5] Confinement in quantum chromodynamics leads to the production of
flux tubes connecting colour charges. The flux tubes act as attractive
QCD string-like potentials.
Timeline
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.[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 in MeV/c2.
In 2004, the DsJ(2632) state seen in
Fermilab's SELEX was suggested as a possible tetraquark candidate.[8]
In 2007, Belle announced the observation of the
Z(4430) state, a c c d u tetraquark candidate. There are also indications that the
Y(4660), also discovered by Belle in 2007, could be a tetraquark state.[9]
In 2009,
Fermilab announced that they have discovered a particle temporarily called
Y(4140), which may also be a tetraquark.[10]
In June 2013, the
BES III experiment in China and the Belle experiment in Japan independently reported on
Zc(3900), the first confirmed four-quark state.[13]
In February 2016, the
DØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying to B0 s π± .[16]
In December 2017, DØ also reported observing the X(5568) using a different B0 s final state.[17]
However, it was not observed in searches by the LHCb,[18]CMS,[19]CDF,[20] or
ATLAS[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 a
c c c c
tetraquark: X(6900).[2][25] In 2022, ATLAS observed X(6900).[26]
In 2021, LHCb announced the discovery of four additional tetraquarks, including ccus.[3]
In 2022, LHCb announced the discovery of csud and csud.[27]