From symmetry to dynamics in modern particle physics

Abstract

Symmetry plays a double role in modern particle physics. As global internal symmetry, it gives structure to the wealth of experimental data concerning quarks and leptons. As gauged internal symmetry, it fixes the dynamical content of three of the four basic interactions in nature: electromagnetic, strong and weak. In electromagnetism, the one global internal symmetry is that of “electric charge,” and its gauging led—by 1950—to the highly successful (Abelian) U(1) gauge theory of electromagnetism—quantum electrodynamics (or QED for short). It took several more decades to identify the correct global internal symmetries in the strong and weak interactions to be gauged. For the strong interaction (among the quarks) the gauging of the three “color” internal degrees of freedom associated with each quark produced the phenomenologically correct (non-Abelian) SU(3) color gauge theory of the strong interaction—quantum chromodynamics (or QCD for short). For the weak interaction (among quarks and leptons) the proper choice was the seven chiral quark and lepton flavors in each generation and their gauging gave rise to the equally successful (non-Abelian) SU(2) × U(1) dynamical theory of the electroweak interaction—quantum flavordynamics (or QFD for short). The remarkable triumphs of the gauge theories of QED, QCD and QFD are testimony to the power of the old Einsteinian dictum: “symmetry dictates dynamics.”