Low-Scale Seesaw and the CP Violation in Neutrino Oscillations

Abstract

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 ${\nu }_{1R}$ and ${\nu }_{2R}$ having a Majorana mass term with mass $M$, which conserves the lepton charge $L$. The RH neutrino ${\nu }_{2R}$ has lepton-charge conserving Yukawa couplings $g_{\ell 2}$ to the lepton and Higgs doublet fields, while small lepton-charge breaking effects are assumed to induce tiny lepton-charge violating Yukawa couplings $g_{\ell 1}$ for ${\nu }_{1R}$, $l=e,\mu ,\tau$. In this approach the smallness of neutrino masses is related to the smallness of the Yukawa coupling of ${\nu }_{1R}$ 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 $|g_{\ell 2}|$ can be much larger than $|g_{\ell 1}|$, of the order $|g_{\ell 2}|\sim 10^{-4}-10^{-2}$, 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 $\delta$ is predicted to have approximately one of the values $\delta \simeq \pi /4,3\pi /4$, or $5\pi /4,7\pi /4$, 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.

Comments: 29 pages, 3 figures, 3 tables; typos corrected; matches published version