HIGGS SCALAR IN THE GRAND DESERT WITH OBSERVABLE PROTON LIFETIME IN SU(5) AND SMALL NEUTRINO MASSES IN SO(10)

We find that the presence of a real scalar in the grand desert transfo rming as zeta(3,0,8) under SU(2)L X U(1)Y X SU(3)C ensures the agreeme nt of the GUT predictions with the data from CERN LEP and proton lifet ime (tau(p)). The mass of zeta is predicted to be close to the Peccei- Quinn symmetry-breaking scale. The computation of the threshold effect s in SU(5) with Higgs representations 24, 5, and 75 shows that the max imum allowed tau(p) for reasonable superheavy Higgs boson masses is ac cessible to experimental tests at low energies. The additional predict ions in SO(10) are small neutrino masses compatible with solutions to the solar-neutrino problem and the dark matter of the Universe.