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 ν 1 R and ν 2 R having a Majorana mass term with mass M , which conserves the lepton charge L . The RH neutrino ν 2 R has lepton-charge conserving Yukawa couplings g ℓ 2 to the lepton and Higgs doublet fields, while small lepton-charge breaking effects are assumed to induce tiny lepton-charge violating Yukawa couplings g ℓ 1 for ν 1 R , l = e , μ , τ . In this approach the smallness of neutrino masses is related to the smallness of the Yukawa coupling of ν 1 R 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 | g ℓ 2 | can be much larger than | g ℓ 1 | , of the order | g ℓ 2 | ∼ 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 , 3 π / 4 , or 5 π / 4 , 7 π / 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. [ABSTRACT FROM AUTHOR]