More missing VEV mechanism in supersymmetric SO(10) model
  The anomalous gauge $U(1)_A$ symmetry which could emerge in the context of the string theories can be very useful ingredient towards builting the complete supersymmetric SO(10) model. We present an example of the $SO(10)\times U(1)_A$ model which provides the ``all order'' solution to the doublet-triplet splitting problem via the missing VEV mechanism -- the Planck scale corrections only can induce the mu-term naturally of order 1 TeV. An interesting feature of this model is that all relevant GUT scale VEVs are defined by the single dimensional parameter in the Higgs superpotential, so that the SO(10) symmetry breaks down to the MSSM practically at one step, without intermediate stages. The colour Higgsino mediated d=5 operators can be naturally suppressed. We also extend the model by implementing $U(1)_A$ as a horizontal symmetry for explaining the fermion mass and mixing pattern, and obtain a predictive texture for fermion masses. This model implies a moderate value of $\tan\beta$ ($\sim 6-10$) and leads to five predictions for the low energy observables. It also leads to the neutrino masses and mixing pattern that could naturally explain both the atmospheric and solar neutrino problems. In addition, a remarkable interplay of the SO(10) and $U(1)_A$ symmetries guarantees an automatic R parity conservation at any order in $M_P^{-1}$, and also suppresses the Planck scale induced B and L violating d=5 operators to the needed level. 
Zurab Berezhiani and Zurab Tavartkiladze (INFN Ferrara)