Baryons are the family of composite particle made of three quarks, as opposed to the mesons which are the family of composite particles made of one quark and one antiquark. Both baryons and mesons are part of the larger particle family comprising all particles made of quarks – the hadrons. 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 masses than other particles.
Until very recently, it was believed that some experiments showed the existence of pentaquarks – "exotic" baryons made of four quarks and one antiquark.[1][2] The particle physics community as a whole did not view their existence as likely in 2006,[3] and in 2008, considered evidence to be overwhelmingly against the existence of the reported pentaquarks.[4]
Since baryons are composed of quarks, they participate in the strong interaction. Leptons on the other hands, are not composed of quarks and as such do not participate in the strong interaction. The most famous baryons are the protons and neutrons which make up most of the mass of the visible matter in the universe, whereas electrons (the other major component of atoms) are leptons. Each baryon has a corresponding antiparticle (antibaryon) where quarks are replaced by their corresponding antiquarks. For example, a proton is made of two up quarks and one down quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.
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, are hadrons, meaning they are particles composed of quarks. Quarks have baryon numbers of B = 1⁄3 and antiquarks have baryon number of B = −1⁄3. The term "baryon" usually refers to triquarks—baryons made of three quarks (B = 1⁄3 + 1⁄3 + 1⁄3 = 1). Other "exotic" baryons have been proposed, such as pentaquarks — baryons made of four quarks and one antiquark (B = 1⁄3 + 1⁄3 + 1⁄3 + 1⁄3 − 1⁄3 = 1), but their existence is not generally accepted. Theoretically, heptaquarks (5 quarks, 2 antiquarks), nonaquarks (6 quarks, 3 antiquarks), etc. could also exist.
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, as implied by the name, is 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 because 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).
