Standard Model

• With the invention of particle accelerators, physicists discovered many new, tiny fundamental particles that were the building blocks of protons, neutrons and electrons. In 1964, the quark concept was proposed by Murray Gell-Mann and George Zweig to explain these particles and their interactions through the strong force. This theory evolved into the now widely accepted Standard Model of fundamental particles.
  
  

Quark Types

• Later in 1964, Gell-Mann and Zweig proposed that three types of particle composites of quarks and antiquarks, called up, down and strange quarks, had spin and electric charges. While studying lepton patterns in 1964, Sheldon Glashow and James Bjorken discovered the fourth quark flavor, charm. In 1965, O.W. Greenburg, M.Y. Han and Yoichiro Nambu proposed quark color change properties.
  
  

Evidence of Quarks

• By 1966, quark theory is not universally accepted due to lack of observational evidence of the existence of quarks. From 1968 to 1969 at the Stanford Linear Accelerator, James Bjorken and Richard Feynman bounced electrons off tiny particles inside protons. This experiment provided the first physical evidence for quarks.