Consistency checks of grand unified theories

The world averaged values of the electroweak and strong couplings and the lower limits on the proton lifetime are used for consistency checks of grand unified theories (GUTs). It is confirmed that new physics outside the standard model (SM) is required to obtain unification of the electroweak and strong forces. Such new physics could come from the minimal supersymmetric extension of the SM, which provides unification consistent with the present limits on the proton lifetime, but also non-supersymmetric models based on additional split multiplets show similar unification properties. The results can be summarized as constraints on the slope differences of the coupling constants. These conditions have to be satisfied by any unified model having a single threshold at about 1 TeV, but cannot distinguish between them. Only future experiments will be able to do so