Missing doublet model reexamined

We reexamine the missing doublet model (MDM) as a means to address the apparent difficulties of the minimal SU(5) supergravity model in dealing with the doublet-triplet splitting problem, the prediction of α3(MZ), and the proton lifetime. We revamp the original MDM by extending its observable sector to include fields and interactions that naturally suppress the dimension-five proton decay operators and that allow seesaw neutrino masses. We also endow the model with a hidden sector which (via gaugino condensation) triggers supersymmetry breaking of the desired magnitude, yields (via hidden matter condensation) a new dynamical intermediate scale for the right-handed neutrino masses (∼1010 GeV), and provides an effective Higgs mixing parameter μ. The model is consistent with gauge coupling unification for experimentally acceptable values of α3(MZ), and with proton decay limits even for large values of tanβ. The right-handed neutrinos can be produced subsequent to inflation, and their out-of-equilibrium decays induce a lepton asymmetry which is later reprocessed (via sphaleron interactions) into a baryon asymmetry at the electroweak scale. The resulting seesaw neutrino masses provide a candidate for the hot dark matter component of the Universe (mντ∼10eV) and are consistent with the MSW solution to the solar neutrino problem. We finally compare the features of this traditional GUT model with that of the readily string-derivable SU(5)xU(1) model, and discuss the prospects of deriving the revamped MDM from string theory. copyright 1996 The American Physical Society