Quark
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.
Discovery of the neutron
The discoveries of the neutron and positron in 1932 were the start of the discoveries of many new particles. Muons were discovered in 1936. Pions and kaons were discovered in 1947, while lambda particles were discovered in 1950. Throughout the 1950s and 1960s, a large number of particles called hadrons were discovered. A classification scheme for organizing all these particles, proposed independently by Murray Gell-Mann and
George Zweig in 1964, became known as the quark model. By this model, particles such as the proton and neutron were not elementary, but composed of various configurations of a small number of other truly elementary particles called partons or quarks. The quark model received experimental verification beginning in the late 1960s and finally provided an explanation for the neutron's anomalous magnetic moment.