Abstract

The natural conservation of flavours to O( G_F²) in neutral weak interactions severely constrains choices of gauge groups as well as their fermion representations. In the absence of exactly conserved quantum numbers other than charge, and of | ΔQ| ⩾ 2 charged currents, essentially the only weak and electromagnetic gauge groups whose neutral interactions naturally conserve all flavours are SU(2) _L ⊗ U(1) and SU(2) _L ⊗ [U(1)] ². The plausible extensions of these gauge groups to grand unified models including the strong interactions are based on SU(5) and SO(10) respectively. Making the SU(5) model completely natural, including in the Higgs sector, gives the prediction m _d/m _e ⋍ m _s/m _μ ⋍ m _b/m _τ ⋍ 2605 where τ is the probable new heavy lepton and b is the conjectured third flavour of charge {-1}/{3}quark. The SO(10) model contains a potential SU(2) _L ⊗ SU(2) _R ⊗ U(1) weak and electromagnetic gauge group, and has a complicated Higgs structure which does not naturally conserve quark flavours.