University of David
Contemporaneously with the evolution of EWT, physicists made substantial progress toward a renormalizable Yang-Mills theory of the strong force. In 1961, Murray Gell-Mann and, independently, Yuval Ne’eman proposed a scheme that organized hadrons into families on the basis of parity and spin. The “Eightfold Way”, as Gell-Mann dubbed the classification, succeeded spectacularly in predicting the Ú− baryon, which was detected in a bubble chamber at Brookhaven National Laboratory (BNL) in 1964. The realization that hadrons could be grouped according to a pattern that was both descriptive and predictive was a major step forward, but physicists sensed that there were too many hadrons for them all to be fundamental. In 1964,Gell-Mann and, independently, George Zweig explained SU(3) symmetry of the Eightfold Way by introducing a smallnumber of fractionally charged building blocks which, when combined in certain pairs and triplets, would yield hadronic matter. These constituents, named quarks and antiquarks by Gell-Mann, remained entirely hypothetical until 1968, when a team at SLAC used James Bjorken’s theory of deep inelastic scattering to demonstrate that nucleons have substructure.2 Even then, the quark model presented a serious problem. Inside a nucleon, quarks seemed to act as free particles. However, in the deep inelastic scattering of electrons from nucleons, no free quark had ever been pried loose. This concern was resolved in 1973 when David Gross and Frank Wilczek, and, separately, David Politzer, demonstrated that a Yang-Mills theory can enjoy a property known as asymptotic freedom: in QCD, as distance becomes vanishingly small, so does the strong force. The analogy of an elastic band that holds quarks inside a hadron is sometimes made.