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, new hadrons were discovered and the "
particle zoo" grew from a few particles in the early 1930s and 1940s to several dozens of them in the 1950s. Some particles were much longer lived than others; most particles decayed through the
strong interaction and had
lifetimes of around 10−23 seconds. When they decayed through the
weak interactions, they had lifetimes of around 10−10 seconds. While studying these decays,
Murray Gell-Mann (in 1953) and
Kazuhiko Nishijima (in 1955) developed the concept of
strangeness (which Nishijima called
eta-charge, after the
eta meson ()) to explain the "strangeness" of the longer-lived particles. The
Gell-Mann–Nishijima formula is the result of these efforts to understand strange decays. Despite their work, the relationships between each particle and the physical basis behind the strangeness property remained unclear. In 1961, Gell-Mann and
Yuval Ne'eman independently proposed a hadron classification scheme called the
eightfold way, also known as
SU(3) flavor symmetry. This ordered hadrons into
isospin multiplets. The physical basis behind both isospin and strangeness was only explained in 1964, when Gell-Mann and
George Zweig independently proposed the
quark model, which at that time consisted only of the up, down, and strange quarks. Up and down quarks were the carriers of isospin, while the strange quark carried strangeness. 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 identify the three-bodies as quarks, instead preferring
Richard Feynman's
parton description, but over time the quark theory became accepted (see
November Revolution). == See also ==