The story of quarks really begins in the mid 1950's with the classical experiments on the proton's electromagnetic structure. This work, done by Robert Hofstadter and his group at Stanford, showed that a proton has a finite size (a root‐mean‐square charge radius of 0.8 fermi), rather than being a point particle, as is an electron. In the early 1960's, the emerging spectra of mesons and baryons led theorists to organize hadrons according to various symmetry schemes. The most successful of these, then and to the present time, has been the so‐called “SU(3)” scheme. In an attempt to provide a physical basis for the SU(3) symmetry, Murray Gell‐Mann and George Zweig proposed the concept of quarks, which were (are) presumed to be the physical manifestations of the SU(3) parameters and hence the structural constituents of hadrons.
The lack of positive evidence for physical particles with fractional charge has not halted attempts to find them with accelerators, in cosmic rays or in stable matter.