Threshold effects on intermediate mass and proton lifetime predictions in SU(5) with split multiplets

Including two-loop contributions to the renormalization group equations (RGE's) and threshold modification of the GUT boundary condition for the gauge couplings, we derive analytic formulas for the mass scales and the GUT coupling constant in the SU(5) split-multiplet models recently investigated by Amaldi, de Boer, Frampton, Furstenau, and Liu. The mass scales and the coupling constants are computed using these analytic formulas in conjuction with the iterative convergence procedure adopted for the solutions of RGE's. In each model the threshold effects are evaluated under the assumption that the superheavy masses are degenerate, partially degenerate, or nondegenerate, resulting in a very significant modification of the degenerate intermediate mass of the new particles. The threshold-uncertainty factor is noted to be the least in the partially degenerate case in one of the eight models making the GUT predictions more precise on the intermediate mass. But the corrections are larger both on the proton lifetime and the intermediate mass in the nondegenerate case in all models. For reasonably precise model predictions we conclude that the superheavy masses may be either degenerate, or partially degenerate, or else their mass-splitting coefficient may be small in the nondegenerate case