Baryon

In particle physics, the baryons are the family of subatomic particles which are made of three quarks. The family notably includes the proton and neutron, which make up the atomic nucleus, but many other unstable baryons exist as well. The term "baryon" is derived from the Greek βαρύς (barys), meaning "heavy," because at the time of their naming it was believed that baryons were characterized by having greater mass than other particles.

Background

Combinations of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet.

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 such that the total spin is 3/2. They primarily decay into a pion and either a proton or neutron.

Lambda baryons (Λ⁰) are composed of one up, one down, and one strange quark, with the up and down quarks in an isospin 0 (flavor-antisymmetric) state. The neutral lambda provided the first observational evidence of the strange quark. In almost all cases a lambda decays to a proton and a charged pion, or to a neutron and a neutral pion.

Sigma baryons (Σ⁺, Σ⁰, Σ⁻), are also composed of one strange quark and a combination of up and down quarks, but arranged in an isospin 1 state. The neutral sigma has the same quark composition as the 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. They decay predominantly into a lambda and a pion; the lambda subsequently decays as described above. Because of this cascading sequence of decays, a Ξ state is sometimes referred to as a cascade.

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.

There are additional baryon states which contain heavy quarks. These are denoted by the Greek letter corresponding to their light (up/down/strange) flavor content with a subscript indicating that a strange quark should be replaced by a heavier quark. For example, the Λ⁺c has quark content (charm, up, down) instead of (strange, up, down). (See: charmed baryons, bottom baryons.)

The octet of light spin-1/2 baryons.

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 our bodies). 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 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 issue 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).

See also

* List of baryons
* Baryon number
* Particle physics
* Pentaquark
* List of particles
* Proton decay