U(1) flavor symmetry and proton decay in supersymmetric standard model

Issues of proton decay are examined in supersymmetric standard model with U(1) flavor symmetry. Dimension five proton-decay operators which arise generically are controlled by the flavor symmetry. We show that unlike the minimal supersymmetric SU(5) case the proton decay modes containing charged lepton can have large branching ratios if the dimension five operators of left-handed type dominate. Measuring the branching ratio of the electron mode to the muon mode may reveal the mechanism of the neutrino mass generation. The case with vanishing charges for Higgs doublets marginally survives the present experimental bound on proton life time.