Anomalous U(1) symmetry and missing doublet SU(5) model

We present the supersymmetric $SU(5)$ models which provide a simple ``all order'' solution to the doublet-triplet splitting problem through the missing doublet mechanism. The crucial role is played by the anomalous $U(1)_A$ gauge symmetry and no additional discrete or global symmetries are needed. Remarkably, such models can be realized even if the 75-plet Higgs is replaced by the standard 24-plet. The same $U(1)_A$ symmetry can also guarantee an exact or approximate conservation of R parity, by suppressing the B and L violating operators to the needed level. The neutrino masses and the proton decay via $d=5$ operators are also examined. We also extend the model by incorporating $U(1)_A$ as a horizontal symmetry for explaining the fermion mass and mixing hierarchy. Interestingly, in this scheme the necessary mild violation of the troublesome $SU(5)$ degeneracy between the down quark and the charged lepton masses can be induced by certain R-parity violating operators.