quark

quark (kwôrk), any of a group of elementary particles that are the basic constituents of all hadrons. Quarks have fractional charges of 1/3 or 2/3 of the basic charge of the electron or proton. There is evidence for six kinds, or flavors, of quarks: up, down, strange, charm, and bottom; the discovery of evidence for the existence of the sixth, labeled top, was announced in 1994. Each flavor of quark is believed to come in three varieties, differing in a property called color. The baryons, a subgroup of the hadrons that includes the proton and neutron, consist of three quarks. A proton consists of two up quarks and a down quark, and a neutron of two down quarks and an up quark. The quarks in the proton and neutron are held together by a cloud of gluons which holds them together and significantly increases the mass. Three antiquarks make up the antibaryons. Mesons, the other subgroup, consist of a quark-antiquark pair.