Quantum chromodynamics possess an additional SU(3) gauge degree of freedom, later called color charge. Han and Nambu noted that quarks might interact via an octet of vector gauge bosons: the gluons. Since free quark searches consistently failed to turn up any evidence for the new particles, and because an elementary particle back then was "defined" as a particle which could be separated and isolated, Gell-Mann often said that quarks were merely convenient mathematical constructs, not real particles. The meaning of this statement was usually clear in context: He meant quarks are confined, but he also was implying that the strong interactions could
Gauge theory Lagrangian. In the quantized version of the obtained classical field theory, the quanta of the gauge field "A"("x") are called gauge bosons. The interpretation of the interaction Lagrangian in quantum field theory is of scalar bosons interacting by the exchange of these gauge bosons. The picture of a classical gauge theory developed in the previous section is almost complete, except for the fact that to define the covariant derivatives "D", one needs to know the value of the gauge field formula_29 at all space-time points. Instead of manually specifying the values of this field, it can be given as the