THRESHOLD AND COMPACTIFICATION EFFECTS IN GUTS

We review general results on threshold effects and their implications on GUTs in the context of LEP data. Among the blooming grand-desert mo dels, threshold effects are computed in the presence of a single real scalar zeta(3, 0, 8) with M zeta similar or equal to 10(10) GeV leadin g to experimentally testable predictions on the proton lifetime tau(p) in SU(5) and, in addition, small neutrino masses in SO(10) needed for the solar neutrino flux and the dark matter of the universe. The fine structure constant matching at M(Z) is ensured by including threshold effects on the unification coupling. In the minimal SUSY SU(5) such e ffects at the GUT scale modify the prediction of the supersymmetric ma ss threshold near the TeV scale and the precision measurments of the S tandard Model couplings at M(Z) probe into the superheavy mass spectru m. Consequences of theorems proved very useful for threshold, compacti fication and multiloop effects are discussed. It is noted that in a cl ass of GUTs the highest intermediate scale M(I) above which G(224P) be comes a good symmetry is not affected by the GUT threshold or compacti fication effects or multiloop contributions in the range M(I) - M(U). But spontaneous compatification effects can decrease the intermediate scale drastically in models where parity and SU(2)(R) breakings are de coupled. Low mass W-R-bsosns are permitted in models with decoupled pa rity and SU(2)(R) breakings,