In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). The term "baryon" is derived from the Greek barys, meaning "heavy," as they are heavier than the other main groups of particles.

Baryons are strongly interacting fermions — that is, they experience the strong nuclear force and are described by Fermi-Dirac statistics, which apply to all particles obeying the Pauli exclusion principle. This is in contrast to the bosons, which do not obey the Exclusion principle.

Baryons, along with mesons, belong to the family of particles known as hadrons, meaning they are composed of quarks. Baryons are fermions composed of three quarks, while mesons are bosons composed of a quark and an antiquark. The quark model classification of baryons is based on this construction.

In addition to the nucleons (protons and neutrons), other members of the baryon family include the Δ, Λ, Σ, Ξ and Ω particles.

Delta baryons (Δ++, Δ+, Δ0, Δ) are composed of a combination of up and down quarks, and decay into a pion and either a proton or neutron.

Lambda baryons (Λ0, Λ+c) are composed of one up, one down, and either a charm or a strange quark. The neutral lambda provided the first observational evidence of the strange quark.

Sigma baryons (Σ+, Σ0, Σ), are composed of one strange quark and a combination of up and down quarks. The neutral sigma has the same quark composition as the neutral lambda (up, down, strange), and so decays much faster than either Σ+ (up, up, strange) or Σ (down, down, strange).

Xi baryons, (Ξ0, Ξ), are composed of two strange quarks and either an up or down quark. The neutral xi, Ξ0, composed of an up and two strange quarks, decays into a neutral lambda and a neutral pion, which itself rapidly decays into an electron and a positron; these immediately annihilate, and so it appears that the xi's product is a lambda that is emitting gamma rays.

The omega minus baryon (Ω) is composed of three strange quarks. Its discovery was a great triumph in the study of quark processes, since it was found only after its existence, mass, and decay products had already been predicted.

Baryonic matter

Baryonic matter is matter composed mostly of baryons (by mass), which includes atoms of any sort (and thus includes nearly all matter that we may encounter or experience in everyday life, including ourselves). Non-baryonic matter is the fundamental antithesis of such matter, being any sort of matter that is not primarily composed of baryons. This might include such ordinary matter as neutrinos, photons or free electrons; however, it may also include exotic species of non-baryonic dark matter, such as supersymmetric particles, axions or black holes. The distinction between baryonic and non-baryonic matter is important in cosmology, because Big Bang nucleosynthesis models set tight constraints on the amount of baryonic matter present in the early universe.

The very existence of baryons is also a significant problem in cosmology, since we have assumed that the Big Bang produced a state with equal amounts of baryons and anti-baryons. The process by which baryons come to outnumber their antiparticles is called baryogenesis (in contrast to a process by which leptons account for the predominance of matter over antimatter, leptogenesis).

A table of baryons

Particle Symbol Makeup Rest mass
B S C Mean lifetime
Proton p uud 938.3 +1 0 0 Stable1
Neutron n ddu 939.6 +1 0 0 9202
Delta Δ++ uuu 1232 +1 0 0 6×10-24
Delta Δ+ uud 1232 +1 0 0 6×10-24
Delta Δ0 udd 1232 +1 0 0 6×10-24
Delta Δ- ddd 1232 +1 0 0 6×10-24
Lambda Λ0 uds 1115.7 +1 -1 0 2.60×10-10
Lambda Λ+c udc 2285 +1 0 1 2.0×10-13
Sigma Σ+ uus 1189.4 +1 -1 0 0.8×10-10
Sigma Σ0 uds 1192.5 +1 -1 0 6×10-20
Sigma Σ- dds 1197.4 +1 -1 0 1.5×10-10
Xi Ξ0 uss 1315 +1 -2 0 2.9×10-10
Xi Ξ- dss 1321 +1 -2 0 1.6×10-10
Omega Ω- sss 1672 +1 -3 0 0.82×10-10

1at least 1035 years. See Proton decay.
2for free neutrons; in most common nuclei, neutrons are stable.

See also

Particles in Physics - Composite particles

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