We study consequences of the threshold effects of supersymmetric (SUSY) and superheavy (GUT) particles to the gauge coupling unification condition in two specific supersymmetric SU(5) models, the minimal model and the missing doublet model with natural doublet-triplet splitting. We present a consistent treatment of the SU(2)×U(1) breaking mass terms in the SUSY particle threshold effects, as well as that of the top quark threshold effect, which have been ignored in previous works. The GUT threshold effects are constrained by the proton decay experiments and by some theoretical consistency conditions, but they are strongly model dependent. For example, under a certain assumption for the SUSY particle masses, the minimal model favors a large (>1 TeV) SUSY breaking scale or a high αs(mz)(<0.12), whereas the missing doublet model allows a low (<1 TeV) SUSY breaking scale for 0.11>αs(mz)>0.13. The consequences of these two models in the proton decay experiments are also briefly discussed.
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