Low-scale seesaw and the CP violation in neutrino oscillations

We consider a version of the low-scale type I seesaw mechanism for generating small neutrino masses, as an alternative to the standard seesaw scenario. It involves two right-handed (RH) neutrinos ν1R and ν2R having a Majorana mass term with mass M, which conserves the lepton charge L. The RH neutrino ν2R has lepton-charge conserving Yukawa couplings gℓ2 to the lepton and Higgs doublet fields, while small lepton-charge breaking effects are assumed to induce tiny lepton-charge violating Yukawa couplings gℓ1 for ν1R, l=e,μ,τ. In this approach the smallness of neutrino masses is related to the smallness of the Yukawa coupling of ν1R and not to the large value of M: the RH neutrinos can have masses in the few GeV to a few TeV range. The Yukawa couplings |gℓ2| can be much larger than |gℓ1|, of the order |gℓ2|∼10−4–10−2, leading to interesting low-energy phenomenology. We consider a specific realisation of this scenario within the Froggatt–Nielsen approach to fermion masses. In this model the Dirac CP violation phase δ is predicted to have approximately one of the values δ≃π/4,3π/4, or 5π/4,7π/4, or to lie in a narrow interval around one of these values. The low-energy phenomenology of the considered low-scale seesaw scenario of neutrino mass generation is also briefly discussed.