Probing the grand-unification-scale mass spectrum through precision measurements on the weak-scale parameters

We study the renormalization-group evolution of gauge coupling constants including mass threshold effects of both light and heavy particles, in the minimal supersymmetric SU(5) grand unified model assuming supersymmetry breaking at {}1 TeV. We show that a certain combination of superheavy masses MV²{M}_{{{Σ}}}$ is tightly constrained, while the mass of the color-triplet Higgs boson, ${M}H, can vary from 2×{10}¹³to 2×{10}¹⁷$ GeV. With a relatively heavy color-triplet mass (${M}H~{10}¹⁷$ GeV), the present nucleon-decay experiments still allow superparticles below 1 TeV. The importance of a precise measurement on ${{{α}}}₃$ is stressed.