hyperon

hyperon (hī`pərŏn'), class of elementary particles heavier than nucleons (proton and neutron). The nucleons and the hyperons together make up the baryon family of particles.

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hyperon [′hī·pə‚rän]

(particle physics)

An elementary particle which has baryon numberB= + 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^-22second).

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Hyperon

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