Seesaw model with softly broken L_e - L_\mu - L_\tau

Abstract

We enlarge the lepton sector of the Standard Model by adding to it two right-handed neutrino singlets, and propose a model based on the seesaw mechanism and on the lepton number $\bar L = L_e - L_\mu - L_\tau$. The model is innovative from the theoretical point of view in that the $\bar L$-invariant mass term of the right-handed neutrinos is associated with the large mass scale responsible for the seesaw mechanism, whereas $\bar L$ is softly broken at a smaller scale by the remaining Majorana mass terms of the right-handed neutrinos. While being very economical, the model has predictive power: one neutrino is massless ($m_3 = 0$), the lepton mixing matrix $U$ features $U_{e3} = 0$, and the solar-neutrino oscillations have maximal amplitude.