Baryon

In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles, which are the quark-based particles. They are also classified as fermions, i.e., they have half-integer spin. In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles, which are the quark-based particles. They are also classified as fermions, i.e., they have half-integer spin. The name 'baryon', introduced by Abraham Pais, comes from the Greek word for 'heavy' (βαρύς, barýs), because, at the time of their naming, most known elementary particles had lower masses than the baryons. Each baryon has a corresponding antiparticle (antibaryon) where their corresponding antiquarks replace quarks. 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. As quark-based particles, baryons participate in the strong interaction, which is mediated by particles known as gluons. The most familiar baryons are protons and neutrons, both of which contain three quarks, and for this reason these particles are sometimes described as triquarks. These particles make up most of the mass of the visible matter in the universe, as well as forming the components of the nucleus of every atom. Electrons (the other major component of the atom) are members of a different family of particles, known as leptons, which do not interact via the strong force. Exotic baryons containing five quarks (known as pentaquarks) have also been discovered and studied. Baryons are strongly interacting fermions; that is, they are acted on by 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, particles composed of quarks. Quarks have baryon numbers of B = 1/3 and antiquarks have baryon numbers 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. The particle physics community as a whole did not view their existence as likely in 2006, and in 2008, considered evidence to be overwhelmingly against the existence of the reported pentaquarks. However, in July 2015, the LHCb experiment observed two resonances consistent with pentaquark states in the Λ0b → J/ψK−p decay, with a combined statistical significance of 15σ.