SPONTANEOUS SYMMETRY BREAKING IN PARTICLE PHYSICS: A CASE OF CROSS FERTILIZATION

Abstract

I will begin by a short story about my background. I studied physics at the University of Tokyo. I was attracted to particle physics because of the three famous names, Nishina, Tomonaga and Yukawa, who were the founders of particle physics in Japan. But these people were at dieren t institutions than mine. On the other hand, condensed matter physics was pretty good at Tokyo. I got into particle physics only when I came back to Tokyo after the war. In hindsight, though, I must say that my early exposure to condensed matter physics has been quite benecial to me. Particle physics is an outgrowth of nuclear physics which began in the early 1930’s with the discovery of the neutron by Chadwick, the invention of the cyclotron by Lawrence, and the \invention of meson theory by Yukawa. a The appearance of an ever increasing array of new particles in the subsequent decades, and the advances in quantum eld theory gradually led to our understanding of the basic laws of nature, culminating in the present standard model. When we faced those new particles, our rst attempts were to make sense out of them by nding some regularities in their properties. They invoked the symmetry principle to classify them. A symmetry in physics leads to a conservation law. Some conservation laws are exact, like energy and electric charge, but these attempts were based on approximate similarities of masses and interactions. Nevertheless, seeing similarities is a natural and very useful trait of the human mind. The near equality of proton and neutron masses and their interactions led to the concept of isospin SU(2) symmetry. 2 On the other hand, one could also go in the opposite direction, and elevate a symmetry to a more elaborate gauged symmetry. Then symmetry will determine the dynamics as well, a most attractive possibility. Thus the beautiful properties of electromagnetism was extended to the