ABSTRACT

At the end of Chapter 12, we left the enterprise of constructing a unied theory of fundamental particles and their interactions in a rather unsatisfactory state. As judged by its ability to reproduce the observed phenomena of particle physics, the standard model is outstandingly successful, but it leaves many questions unanswered. There are twenty or so parameters (coupling constants and masses) whose values cannot be deduced from any principles of the theory and must simply be adjusted to t the facts. Likewise, the gauge symmetry group SU(3)SU(2)U(1) and the number of families of quarks and leptons must be chosen, from the limitless possibilities that would seem to present themselves, just so as to t the facts. The apparent convergence of the running coupling constants of the standard model to a single value at around 1015 - 1016GeV seems to point towards a more completely unied underlying theory. If this is taken to be a grand unied gauge theory, though, then disturbingly ad hoc measures (such as the ne tuning of some of its parameters) are needed to t the known facts, while other features of the theory (such as its gauge group) cannot be determined because not enough facts are known!