A tetraquark, in particle physics, is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, the modern theory of strong interactions. A tetraquark state is an example of an exotic hadron which lies outside the conventional quark model classification.
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.
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 MeV/c2.
In 2007, Belle announced the observation of the Z(4430) state, a
tetraquark candidate. There are also indications that the Y(4660), also discovered by Belle in 2007, could be a tetraquark state.
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
(5S) meson (a form of bottomonium), a well-defined tetraquark resonance exists.
In February 2016, the DØ experiment reported evidence of a narrow tetraquark candidate, named X(5568), decaying to
. In December 2017, DØ also reported observing the X(5568) using a different
s final state. However, it was not observed in searches by the LHCb, CMS, CDF, or ATLAS experiments.
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- Announcement by LHCb
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