Calculational schemes in GUTs

The renormalization and decoupling properties of a spontaneously broken Grand Unified Theory (GUT) are discussed. Using these properties, the authors have shown how one may derive precisely the conventional GUT predictions taking into account threshold corrections and eliminating renormalization scheme ambiguities. The authors found that the minimal SU(5) GUT predictions are quite restricted, and that the main ambiguities follow from the unknown superheavy Higgs masses and from the experimental error of the QCD scale Λ/sub MS-bar/. The predicted value of sin/sup 2/θ is in a remarkable agreement with the experiment. Also, the predicted m/sub b//m/sub tau/ ratio agrees perfectly with its experimental value when they assume three generations only. The SU(5) predicted central value for the proton lifetime is much smaller than the present experimental limits coming from the proton decay experiments. After eliminating all threshold ambiguities the remaining uncertainties comes mainly from the Λ/sub MS-bar/ and the unknown Higgs masses. The later uncertainties may change the central value of the SU(5) proton lifetime by 10/sup +-2/. Therefore taking all uncertainties in the ''favourable'' direction, the proton decay experiments are not yet in contradiction with the minimal SU(5) GUT prediction. The supersymmetric extension of Grand Unified Theories (SUSY-GUT) are also considered