Active-sterile neutrino mixing in the absence of bare active neutrino   mass
  We investigate a minimal extension of the standard model in which the only new ingredient is the sterile neutrinos. We do not introduce extra Higgs multiplets or high dimensional effective operators to induce mass terms for the active neutrinos, and the model is renormalizable in itself. We show for arbitrary numbers of generations and sterile neutrinos that the independent physical parameters in the leptonic sector are much less than previously anticipated. For instance, with three active and two sterile neutrinos, there are four mixing angles and three CP phases in addition to four non-vanishing neutrino masses. We study phenomenological implications for tritium beta decay, neutrinoless double beta decay and neutrino oscillations. For the most natural see-saw parameters, we find that it is difficult to accommodate in the model the best-fit values of masses and mixing parameters from oscillation data no matter whether we include or not the null short-baseline experiments together with the LSND result. This implies that if the LSND result is confirmed by MiniBooNE, the see-saw parameter region of the model with two sterile neutrinos could be largely excluded. 
Yi Liao (Nankai Univ)