Grand unified theories II

For pt.I see report no. CERN TH-3174 (1981). The inadequacies of the standard SU(3)*SU(2)*U(1) model of the strong, weak, and electromagnetic interactions motivate the construction of grand unified theories (GUTs). The strategy then adopted was to assign the fundamental fermions-quarks and leptons-to 'generations' forming economical representations of a simple grand unifying group G. The observed disparity of the strong and weak coupling constants implies that the group G must be broken down at a very high energy scale. The properties of the running coupling constants in renormalizable field theories enable one to estimate this breaking scale to be around 10 /sup 15/ GeV in simple models such as SU(5). The real tests of GUTs lie, however, with their predictions for new interactions, which can lead to qualitatively new phenomena such as baryon decay or neutrino masses and oscillations. The author discusses how some of these new interactions may have consequences in terrestrial experiments. The important implications for cosmology is also discussed.