Charmed quarks

By now three kinds of quarks were known: up, down, and strange. But the theory predicted that quarks should exist in pairs, with up-down and strange-charmed proposed as a pairing. Using the latest particle accelerators, a hadron that had to contain a charmed quark was created. The third pair of quarks is the top-bottom, for a total of six quarks and six antiquarks (matching the number of leptons)
 Burton Richter, Samuel Chao Chung Ting United States 1974 Charmed quarks

By now three kinds of quarks were known: up, down, and strange. But the theory predicted that quarks should exist in pairs, with up-down and strange-charmed proposed as a pairing. Using the latest particle accelerators, a hadron that had to contain a charmed quark was created. The third pair of quarks is the top-bottom, for a total of six quarks and six antiquarks (matching the number of leptons)