Superheavy spectrum and supersymmetric grand unification

We examine in detail the effect of the superheavy particle spectrum on the claim that recent data from CERN LEP are consistent with a SUSY SU(5) grand unified theory (GUT) whose superparticle spectrum is characterized by a single scale $M_S$ of approximately 1 TeV, and has three couplings unifying at a scale $\mu =M_V$. Using the most recent LEP determination of ${\alpha }_s$, 0.125±0.005, we find that SUSY unification requires that $M_S<100\mathrm{}$ GeV. Larger values of $M_S$ can be accommodated by assuming that $\Sigma$, a multiplet transforming as the adjoint of SU(5), has a mass 8 to 12 orders of magnitude below the GUT scale, with the GUT scale close to or even above the Planck scale. Larger values of ${\alpha }_s$, with unification below the Planck scale, may be accommodated only by breaking the degeneracy of the color octet and weak triplets of the $\Sigma$ multiplet, but this splitting appears in SUSY SU(5) GUT's only as a gravitational effect in the form of nonrenormalizable operators.