A tetraquark, in particle physics, is an exotic
meson composed of four valence quarks.
In principle, a tetraquark state may be allowed
in quantum chromodynamics, the modern theory
of strong interactions.
Any established tetraquark state would be
an example of an exotic hadron which lies
outside the quark model classification.
== History ==
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 have not been yet 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 2004, the DsJ(2632) state seen in Fermilab's
SELEX was suggested as a possible tetraquark
candidate.In 2007, Belle announced the observation
of the Z(4430) state, a ccdu tetraquark candidate.
There are also indications that the Y(4660),
also discovered by Belle in 2007, could be
a tetraquark state.In 2009, Fermilab announced
that they have discovered a particle temporarily
called Y(4140), 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 ϒ(5S) meson (a form of bottomonium),
a well-defined tetraquark resonance exists.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.In 2014, the Large Hadron
Collider experiment LHCb confirmed the existence
of the Z(4430) state with a significance of
over 13.9 σ.In February 2016, the DØ experiment
reported evidence of a narrow tetraquark candidate,
named X(5568), decaying to B0sπ±.
In December 2017, DØ also reported observing
the X(5568) using a different B0s 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(4274), X(4500) and X(4700).
== See also ==
Color confinement
Hadron
Pentaquark
