TABLE OF CONTENTS The Standard Model of fundamental strong, weak and electromagnetic interactions is a gauge theory involving quarks and leptons based on the symmetry group SU color(3) SU L(2) U Y(1). As we have argued, the weak isospin and hypercharge symmetries, or the symmetry groups SU L(2) and U Y(1) are spontaneously broken. As a result, the weak gauge bosons become massive, and the symmetry at low energy reduces to the gauge symmetry of electromagnetism and of color symmetry, namely SU color(3) U Q(1). Ignoring the finer technical points, this is essentially the spirit of the Standard Model. The Standard Model leads to many interesting perturbative predictions, and all of them appear to hold true. In fact, the agreement between experiment and theory is quite remarkable (see previous chapter). Thus, it seems reasonable to conclude that the Standard Model leads to a correct description of fundamental interactions at low energies. However, the Standard Model has many parameters, e.g., masses of the leptons, quarks, gauge bosons, and of the Higgs, various coupling strengths, and the elements of the CKM matrix, with all values seemingly perplexing and ad hoc. Furthermore, the Standard Model does not incorporate gravity, which is another fundamental force that might be expected to unify with the other fundamental forces at some large mass scale.
The recent discovery that neutrinos carry mass implies, in analogy with the situation for quarks and neutral mesons, that neutrinos too can mix. In...
