EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH Consistency Checks of Grand Uni ed 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 Uni ed Theories (GUT). It is con rmed that new physics outside the Standard Model (SM) is required to obtain uni cation of the electroweak and strong forces. Such new physics could come from the minimal supersymmetric extension of the SM, which provides uni cation consistent with the present limits on the proton lifetime, but non-supersymmetric models based on additional split multiplets show similar uni cation properties. The uni cation condition strongly restricts the number of GUT's, but cannot distinguish between them. Only future experiments will be able to do so.