In 1964, two American physicists, Murray Gell-Mann and George Zweig, offered a theory that seemed to do just that. They proposed that protons, neutrons, and many of the new particles being created in particle accelerators were composed of combinations of truly fundamental particles that Gell-Mann called quarks.
Quarks, according to the theory, had one very unusual property: They were never seen alone, but only in combinations of two or three. Another peculiarity was that they carried electric charges that were a fraction—either -1/3 or +2/3—of the fundamental unit of electric charge carried by the proton. When quarks combined to make particles, these fractional charges added up to a single positive or negative charge or to no charge at all.
The theory predicted that only three kinds of quarks—called up, down, and strange—were required to make up all the known subatomic particles. Protons and neutrons were made of up and down quarks. A proton consisted of two up quarks and one down quark; a neutron contained two down quarks and one up quark. The strange quarks were found in several other kinds of heavy particles that are seldom seen in nature but which are often created during high-energy collisions in particle accelerators.
By 1978, high-energy research had created various new particles requiring the existence of a fourth and fifth quark, which were dubbed charmed and bottom. And the evolving quark theory suggested that quarks come in pairs, so physicists concluded that a sixth quark—the top quark—must also exist.