A fundamental particle, of which hadrons are believed to be composed. Quarks have charges that are either +⅔ or −⅓ of the electron's charge, and combine to make elementary particles. Baryons consist of three quarks, and mesons of two. For example, a proton consists of quarks with charges +⅔+⅔ −⅓ = 1 and a neutron consists of three with charges +⅔ −⅔ −⅓ = 0. Eighteen different quarks with various properties are thought to exist, with a corresponding number of antiquarks. An isolated quark has never been observed; quarks appear to exist only in combination as baryons or mesons.
Subjects: Astronomy and Astrophysics.