Exotic hadrons are a subatomic particles composed
of quarks and gluons, but which - unlike "well-known"
hadrons such as protons , neutrons and mesons
- consist of more than three valence quarks.
By contrast, "ordinary" hadrons contain just
two or three quarks.
Hadrons with explicit valence gluon content
would also be considered exotic.
In theory, there is no limit on the number
of quarks in a hadron, as long as the hadron's
color charge is white, or color-neutral.Consistent
with ordinary hadrons, exotic hadrons are
classified as being either fermions, like
ordinary baryons, or bosons, like ordinary
mesons.
According to this classification scheme, pentaquarks,
containing five valence quarks, are exotic
baryons, while tetraquarks (four valence quarks)
and hexaquarks (six quarks, consisting of
either a dibaryon or three quark-antiquark
pairs) would be considered exotic mesons.
Tetraquark and pentaquark particles are believed
to have been observed and are being investigated;
Hexaquarks have not yet been confirmed as
observed.
Exotic hadrons can be searched for by looking
for S-matrix poles with quantum numbers forbidden
to ordinary hadrons.
Experimental signatures for such exotic hadrons
have been seen by at least 2003 but remain
a topic of controversy in particle physics.
Jaffe and Low suggested that the exotic hadrons
manifest themselves as poles of the P matrix,
and not of the S matrix.
Experimental P-matrix poles are determined
reliably in both the meson-meson channels
and nucleon-nucleon channels.
== History ==
When the quark model was first postulated
by Murray Gell-Mann and others in the 1960s,
it was to organize the states known then to
be in existence in a meaningful way.
As quantum chromodynamics (QCD) developed
over the next decade, it became apparent that
there was no reason why only three-quark and
quark-antiquark combinations could exist.
Indeed, Gell-Mann's original 1964 paper alludes
to the possibility of exotic hadrons and classifies
hadrons into baryons and mesons depending
upon whether they have an odd (baryon) or
even (meson) number of valence quarks.
In addition, it seemed that gluons, the mediator
particles of the strong interaction, could
also form bound states by themselves (glueballs)
and with quarks (hybrid hadrons).
Several decades have passed without conclusive
evidence of an exotic hadron that could be
associated with the S-matrix pole.
In April 2014, the LHCb collaboration confirmed
the existence of the Z(4430)−, discovered
by Belle, and demonstrated that it must have
a minimal quark content of ccdu.In July 2015,
LHCb announced the discovery of two particles,
named P+c(4380) and P+c(4450), which must
have minimal quark content ccuud, making them
pentaquarks.
== Candidates ==
There are several exotic hadron candidates:
X(3872) – Discovered by the Belle detector
at KEK in Japan, this particle has been variously
hypothesized to be diquark or a mesonic molecule.
Y(3940) – This particle fails to fit into
the Charmonium spectrum predicted by theorists.
Y(4140) – Discovered by CDF Fermilab in
March 2009 [1].
Y(4260) – Discovered by the BaBar detector
at SLAC in Menlo Park, California this particle
is hypothesized to be made up of a gluon bound
to a quark and antiquark.
Zc(3900) – Discovered by Belle and BES III
Z(4430) – Discovered by Belle and later
confirmed by LHCb with 13.9σ significance
[2]
X(4274) – Observed by LHCb at CERN arXiv:1606.03179
X(4500) – Observed by LHCb at CERN arXiv:1606.03179
X(4700) – Observed by LHCb at CERN arXiv:1606.03179
== See also ==
Exotic meson
Exotic baryon
Tetraquark
Pentaquark
Hexaquark
== 
Notes ==
