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In particle physics, a hadron is a subatomic particle which experiences the strong nuclear force. In current quark models , hadrons are composed of fermions called quarks and antiquarks and of bosons called gluons. The gluons mediate the color force that binds the quarks together. Hadrons are composite particles bound together by the color interaction, and thus are somewhat analogous to atoms, in which particles are bound together by the electromagnetic interaction.
Hadrons are further classified by their spin and quark content.
- Baryons are composed of three quarks or three anti-quarks and have half-odd-integral spin, i.e. they are fermions. They include the nucleons (the proton and the neutron), which are part of normal atomic nuclei, and particles such as the hyperons (e.g. the Δ, Λ, Σ, Ξ, and Ω), which are generally heavier than nucleons, short-lived, and do not normally appear in atomic nuclei.
- Mesons are composed of a quark and an anti-quark and have integral spin, i.e. they are bosons. They include the pions, the kaons, the rhos , the omegas , and many other types of mesons.
- Exotic hadrons do not have the quark numbers of either ordinary baryons or mesons.
- Exotic baryons are composed of more than three quarks (or anti-quarks) odd in number. The first such particles, pentaquarks, are thought to have been discovered recently. They have four quarks and one anti-quark.
- Exotic mesons contain more than one valence quark-antiquark pair. Recently there has been some evidence for the tetraquark, which consists of two valence quark-antiquark pairs.
- Hybrid mesons consist of at least one valence quark-antiquark pair and at least one real (not virtual) gluon.
- Glueballs contain no valence quarks at all, being composed solely of gluons. These states mix strongly with ordinary mesons and are extremely difficult to identify.
As the hadrons are composite quantum systems, they also exist in excited states known as hadronic resonances. Each ground state hadron may have many excited states, and hundreds have been observed in particle experiments. Resonances decay extremely quickly (within about 10−24 s) via strong interactions.
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