In the beginnings of particle physics (first half of the 20th century),
hadrons such as
protons,
neutrons and
pions were thought to be
elementary particles. However, as new hadrons were discovered, the '
particle zoo' grew from a few particles in the early 1930s and 1940s to several dozens of them in the 1950s. The relationships between each of them were unclear until 1961, when
Murray Gell-Mann and
Yuval Ne'eman (independently of each other) proposed a hadron classification scheme called the
Eightfold Way, or in more technical terms,
SU(3) flavor symmetry. This classification scheme organized the hadrons into
isospin multiplets, but the physical basis behind it was still unclear. In 1964, Gell-Mann and
George Zweig (independently of each other) proposed the
quark model, then consisting only of up,
down, and
strange quarks. However, while the quark model explained the Eightfold Way, no direct evidence of the existence of quarks was found until 1968 at the
Stanford Linear Accelerator Center.
Deep inelastic scattering experiments indicated that protons had substructure, and that protons made of three more-fundamental particles explained the data (thus confirming the
quark model). At first people were reluctant to describe the three bodies as quarks, instead preferring
Richard Feynman's
parton description, but over time the quark theory became accepted (see
November Revolution). == Mass ==