E_6 Unification, Doublet-Triplet Splitting and Anomalous U(1)_A Symmetry
  We propose a natural Higgs sector in E_6 grand unified theory (GUT) with anomalous U(1)_A gauge symmetry. In the scenario, the doublet-triplet splitting can be realized while the proton decay via dimension 5 operator is suppressed. The gauge coupling unification is also realized without finetuning. The GUT scale is generally lower than the usual GUT scale 2\times 10^{16} GeV, the proton decay via dimension 6 operator can be seen in near future experiment. The lifetime of nucleon in the model is roughly estimated as \tau_p(p\to e^+\pi^0)\sim 3\times 10^{33} years. It is shown that the Higgs sector is compatible with the matter sector proposed by one of the authors, which reproduces the realistic quark and lepton mass matrices including bi-large neutrino mixing angle. Combining the Higgs sector and the matter sector, we can obtain the totally consistent E_6 GUT. The input parameters are only 8 integer anomalous U(1)_A charges (+3 for singlet Higgs) for the Higgs sector and 3 (half) integer charges for the matter sector. 
Nobuhiro Maekawa and Toshifumi Yamashita