<P> In less than a year, extensions to the Gell - Mann--Zweig model were proposed . Sheldon Lee Glashow and James Bjorken predicted the existence of a fourth flavor of quark, which they called charm . The addition was proposed because it allowed for a better description of the weak interaction (the mechanism that allows quarks to decay), equalized the number of known quarks with the number of known leptons, and implied a mass formula that correctly reproduced the masses of the known mesons . </P> <P> In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center (SLAC) showed that the proton contained much smaller, point - like objects and was therefore not an elementary particle . Physicists were reluctant to firmly identify these objects with quarks at the time, instead calling them "partons"--a term coined by Richard Feynman . The objects that were observed at SLAC would later be identified as up and down quarks as the other flavors were discovered . Nevertheless, "parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks, and gluons). </P> <P> The strange quark's existence was indirectly validated by SLAC's scattering experiments: not only was it a necessary component of Gell - Mann and Zweig's three - quark model, but it provided an explanation for the kaon () and pion (π) hadrons discovered in cosmic rays in 1947 . </P> <P> In a 1970 paper, Glashow, John Iliopoulos and Luciano Maiani presented the so - called GIM mechanism to explain the experimental non-observation of flavor - changing neutral currents . This theoretical model required the existence of the as - yet undiscovered charm quark . The number of supposed quark flavors grew to the current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation could be explained if there were another pair of quarks . </P>

An antibaryon composed of two anti up quarks and one anti down quark would have a charge of