A baryon is a composite particle made of three quarks. Baryons are opposed to mesons which are made of one quark and one antiquark. Both baryons and mesons belong to the hadron family, which are the particles made of quarks. The name "baryon" comes from the Greek word for "heavy" (βαρύς, barys), because at the time of their naming, most known particles had lower masses than the baryons'.
Since baryons are composed of quarks, they participate in the strong interaction. Leptons, on the other hand, are not composed of quarks and as such do not participate in the strong interaction. The most well known 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.
Until recently, it was believed that some experiments showed the existence of pentaquarks—"exotic" baryons made of four quarks and one antiquark. 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.
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