Exotic hadron
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Exotic hadrons are subatomic particles made of quarks (and possibly gluons), but which do not fit into the usual schema of hadrons. While bound by the strong interaction they are not predicted by the simple quark model. That is, these exotics do not have the same quark content as ordinary hadrons: exotic baryons have more than just the three quarks of ordinary baryons and exotic mesons do not have one quark and one antiquark like ordinary mesons. These exotics can be searched for by looking for particles with quantum numbers forbidden to ordinary hadrons. Experimental signatures for exotic hadrons have been seen recently but remain a topic of controversy in particle physics.
[edit] History
When the quark model was first postulated by Murray Gell-Mann and others in the 1960's it was to organize the states then in existence in a meanful way. As Quantum Chromodynamics (QCD) developed over the next decade, however, it became apparent that there was no fundamental reason why only 3-quark and quark-antiquark combinations should exist. In addition it seemed that gluons, the force carrying particles of the strong interaction, should also form bound states by themselves (glueballs) and with quarks (hybrid hadrons). Nevertheless, several decades have passed without discovering an exotic hadron.
[edit] 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(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.
