Supersymmetry and Unification: Heavy Top Was the Key

Abstract

I review the unification of gauge couplings of strong, weak and electro-magnetic interactions. I start by recalling the history of the most important prediction of low-energy supersymmetry: the correct value of the weak mixing angle tied to a large top quark mass. I then turn to the discussion of the present day situation of the minimal supersymmetric Grand Unified Theories based on SU(5) and SO(10) groups, and I show why the minimal SU(5) is in accord with experiment. For the sake of completeness I also summarize the problems and possible solutions of the minimal ordinary SU(5). One version, based on the minimal Georgi-Glashow model, augmented by the adjoint fermion, predicts a light fermion triplet to lie below TeV or so. Its (lepton number violating) decays offer a hope of probing neutrino (Majorana) masses and mixings at the LHC.