Exotic hadrons are 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.
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.
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 .
- 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 
- 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
- F. E. Close (1988). "Gluonic Hadrons". Reports on Progress in Physics. 51 (6): 833–882. Bibcode:1988RPPh...51..833C. doi:10.1088/0034-4885/51/6/002.
J. Belz et al. (BNL-E888 Collaboration) (1996). "Search for the weak decay of an H dibaryon". Physical Review Letters. 76 (18): 3277–3280. arXiv:hep-ex/9603002. Bibcode:1996PhRvL..76.3277B. doi:10.1103/PhysRevLett.76.3277. PMID 10060926.
The theory of quantum chromodynamics imposes no specific limitation on the number of quarks composing hadrons other than that they form color singlet states.
- See Tetraquark
- See "note on non-q qbar mesons" in PDG 2006, Journal of Physics, G 33 (2006) 1.
- R. L. Jaffe and F. E. Low, Phys. Rev. D 19, 2105 (1979). doi:10.1103/PhysRevD.19.2105
- M. Gell-Mann (1964). "A Schematic Model of Baryons and Mesons". Physics Letters. 8 (3): 214–215. Bibcode:1964PhL.....8..214G. doi:10.1016/S0031-9163(64)92001-3.
- LHCb collaboration (7 April 2014). "Observation of the resonant character of the Z(4430)− state". Physical Review Letters. 112 (22): 222002. arXiv:1404.1903. Bibcode:2014PhRvL.112v2002A. doi:10.1103/PhysRevLett.112.222002. PMID 24949760.
R. Aaij et al. (LHCb collaboration) (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λ0
b→J/ψK−p decays". Physical Review Letters. 115 (7): 072001. arXiv:1507.03414. Bibcode:2015PhRvL.115g2001A. doi:10.1103/PhysRevLett.115.072001. PMID 26317714.