Exotic hadron

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.

There have been many observations of so-called 'exotic candidates' experimentally observed, which are particles that don't appear to fit the standard quark model. The first few exotic candidates to be identified include the X(3872), which was discovered by the Belle experiment in Japan; the Y(4260) which was discovered at the BaBar experiment; and the Zc+(3900), which was discovered independently by the BES III experiment in China and the Belle experiment.

In April 2014, the LHCb collaboration confirmed the existence of the Z(4430)−, discovered by the Belle experiment, and demonstrated that it must have a minimal quark content of cd. Since this was the first exotic hadron to have it's quark content experimentally identified, it is also the first unambiguous discovery of an exotic hadron.

In July 2015, LHCb announced the discovery of two particles, named and , which must have minimal quark content cuud, making them pentaquarks.

Candidates

Notes

References

1. Close, F. E.. (1988). "Gluonic Hadrons". Reports on Progress in Physics.
2. Belz, J., et al. (BNL-E888 Collaboration). (1996). "Search for the weak decay of an H dibaryon". [[Physical Review Letters]].
3. See [[Tetraquark]]
4. (2006). "Note on non-q qbar mesons". Journal of Physics G.
5. (1979). "Connection between quark-model eigenstates and low-energy scattering". Physical Review D.
6. Gell-Mann, M.. (1964). "A Schematic Model of Baryons and Mesons". [[Physics Letters]].
7. Choi, S.-K.. (2003-12-23). "Observation of a Narrow Charmoniumlike State in Exclusive B ± → K ± π + π − J / ψ Decays". Physical Review Letters.
8. Coan, T. E.. (2006-04-28). "Charmonium Decays of Y ( 4260 ) , ψ ( 4160 ) , and ψ ( 4040 )". Physical Review Letters.
9. Ablikim, M.. (2013-06-17). "Observation of a Charged Charmoniumlike Structure in e + e − → π + π − J / ψ at s = 4.26 GeV". Physical Review Letters.
10. Liu, Z. Q.. (2013-06-17). "Study of e + e − → π + π − J / ψ and Observation of a Charged Charmoniumlike State at Belle". Physical Review Letters.
11. [[LHCb]] collaboration. (7 April 2014). "Observation of the resonant character of the Z(4430)⁻ state". Physical Review Letters.
12. "Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model".
13. Aaij, R., et al. (LHCb collaboration). (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λ{{su". [[Physical Review Letters]].
14. (2008-05-19). "Search for new charmonium states in the processes e+ e- → J/psi D() D() at sqrt{s} ~ 10.6 GeV". Physical Review Letters.
15. (2010-03-16). "Evidence for a new resonance and search for the Y(4140) in $\gamma \gamma \to \phi J/\psi$". Physical Review Letters.
16. (2017-03-01). "Evidence of Two Resonant Structures in $e^+ e^- \to \pi^+ \pi^- h_c$". Physical Review Letters.