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(hī`pərŏn'), class of elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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 heavier than nucleons (protonproton,
elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons.
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 and neutronneutron,
uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.
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). The nucleons and the hyperons together make up the baryonbaryon
[Gr.,=heavy], class of elementary particles that includes the proton, the neutron, and a large number of unstable, heavier particles, known as hyperons. From a technical point of view, baryons are strongly interacting fermions; i.e.
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 family of particles.
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A collective name for any baryon with nonzero strangeness number s. The name hyperon has generally been limited to particles which are semistable, that is, which have long lifetimes relative to 10-22 s and which decay by photon emission or through weaker decay interactions. Hyperonic particles which are unstable (that is, with lifetimes shorter than 10-22 s) are commonly referred to as excited hyperons. The known hyperons with spin 1/2 ℏ (where ℏ is Planck's constant divided by 2π) are Λ, Σ-, Σ0, and Σ+ with s = -1, and Ξ- and Ξ0, with s = -2, together with the Ω- particle, which has spin 3/2 ℏ and s = -3. The corresponding antihyperons have baryon number B = -1, opposite strangenesse s, and charge Q; they are all known empirically.

There is no deep distinction between hyperons and excited hyperons, beyond the phenomenological definition above. Indeed, the hyperon Ω(1672)- and the excited hyperons Ξ(1530) and Σ(1385), together with the unstable nucleonic states Δ(1236), are known to form a unitary decuplet of states with spin 3/2 ℏ. See Baryon, Elementary particle, Symmetry laws (physics), Unitary symmetry

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


(particle physics)
An elementary particle which has baryon number B = + 1, that is, which can be transformed into a nucleon and some number of mesons or lighter particles, and which has nonzero strangeness number.
A hyperon (as in the first definition) which is semistable (the lifetime is much longer than 10-22 second).
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.