Seesaw model with softly broken Le−Lμ−Lτ

We enlarge the lepton sector of the Standard Model by adding to it tworight-handed neutrino singlets, and propose a model based on the seesawmechanism and on the lepton number $\bar L = L_e - L_\mu - L_\tau$. The modelis innovative from the theoretical point of view in that the $\bar L$-invariantmass term of the right-handed neutrinos is associated with the large mass scaleresponsible for the seesaw mechanism, whereas $\bar L$ is softly broken at asmaller scale by the remaining Majorana mass terms of the right-handedneutrinos. While being very economical, the model has predictive power: oneneutrino is massless ($m_3 = 0$), the lepton mixing matrix $U$ features $U_{e3}= 0$, and the solar-neutrino oscillations have maximal amplitude.Comment: Latex, 7 pages, no figure