Neutrino mass and the Standard Model
  It is pointed out (not for the first time) that the minimal Standard Model, without additional gauge-singlet right-handed neutrinos or isotriplet Higgs fields, allows for nonvanishing neutrino masses and mixing. The required interaction term is nonrenormalizable and violates the global B-L symmetry. The ultimate explanation of this interaction term may or may not rely on grand unification. 
F. R. Klinkhamer
hep-ph hep-ex