Strange quark
The first strange particle (a particle containing a strange quark) was discovered in 1947 (kaons), but the existence of the strange quark itself (and that of the up and down quarks) was only postulated in 1964 by Murray Gell-Mann and George Zweig to explain the Eightfold Way classification scheme of hadrons. The first evidence for the existence of quarks came in 1968, in deep inelastic scattering experiments at the Stanford Linear Accelerator Center. These experiments confirmed the existence of up and down quarks, and by extension, strange quarks, as they were required to explain the Eightfold Way.
Shoichi Sakata
In 1959, Ikeda, Ogawa and Ohnuki
and, independently, Yamaguchi
found out the U(3) symmetry in the Sakata model. The U(3) symmetry provides a mathematical descriptions of hadrons in the eightfold way
idea (1961) of Murray Gell-Mann. Sakata's model was superseded by the quark model, proposed by Gell-Mann and George Zweig in 1964, which keeps the U(3) symmetry, but made the constituents fractionally charged and rejected the idea that they could be identified with observed particles. Still, within Japan, integer charged quark models parallel to Sakata's were used until the 1970s, and are still used as effective descriptions in certain domains.