Framework for Naturally Light Singlet Neutrinos
  The totality of present neutrino data seems to require four light neutrinos, but only three of them can be the neutral components of left-handed lepton doublets. To accommodate one or more naturally light singlet neutrino(s), an extra U(1) gauge factor is proposed to implement an analogous seesaw mechanism which accounts for the light doublet neutrinos. Using the constraints of anomaly cancellation, the property of this U(1) is determined. The most attractive theoretical framework is that of a supersymmetric $SU(3)_C \times SU(2)_L \times U(1)_Y \times U(1)_N$ model already proposed. 
Ernest Ma (UC Riverside)