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 are included in the Lagrangian to ensure its invariance under the local group transformations (called "gauge invariance"). When such a theory is quantized, the quanta of the gauge fields are called "gauge bosons". If the symmetry group is non-commutative, then the gauge theory is referred to as non-abelian gauge theory, the usual example being the Yang–Mills theory. Many powerful theories in physics are described by Lagrangians that are invariant under some symmetry transformation groups. When they are invariant under a transformation identically performed at "every" point in the spacetime in which the physical processes occur, they are said to have