|
A baryon is a composite subatomic particle made up of three quarks (as distinct from mesons, which are composed of one quark and one antiquark). Baryons and mesons belong to the hadron family of particles, which are the quark-based particles. The name "baryon" comes from the Greek word for "heavy" (βαρύς, ''barys''), because, at the time of their naming, most known elementary particles had lower masses than the baryons. As quark-based particles, baryons participate in the strong interaction, whereas leptons, which are not quark-based, do not. The most familiar baryons are the protons and neutrons that make up most of the mass of the visible matter in the universe. Electrons (the other major component of the atom) 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. ==Background== 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'' = and antiquarks have baryon number of ''B'' = −. The term "baryon" usually refers to ''triquarks''—baryons made of three quarks (''B'' = + + = 1). Other exotic baryons have been proposed, such as pentaquarks—baryons made of four quarks and one antiquark (''B'' = + + + − = 1), but their existence is not generally accepted. In theory, heptaquarks (5 quarks, 2 antiquarks), nonaquarks (6 quarks, 3 antiquarks), etc. could also exist. Until recently, it was believed that some experiments showed the existence of pentaquarks—baryons made of four quarks and one antiquark.〔H. Muir (2003)〕〔K. Carter (2003)〕 The particle physics community as a whole did not view their existence as likely in 2006,〔W.-M. Yao ''et al.'' (2006): (Particle listings – Θ+ )〕 and in 2008, considered evidence to be overwhelmingly against the existence of the reported pentaquarks.〔C. Amsler ''et al.'' (2008): (Pentaquarks )〕 However, in July 2015, the LHCb experiment observed two resonances consistent with pentaquark states in the Λ → J/ψKp decay, with a combined statistical significance of 15σ.〔 decays |journal=Physical Review Letters |volume=115 |issue=7 |doi=10.1103/PhysRevLett.115.072001 |bibcode = 2015PhRvL.115g2001A }}〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「baryon」の詳細全文を読む スポンサード リンク
|