Radiative Generation of the LMA Solution from Small Solar Neutrino Mixing at the GUT Scale

We show that in see-saw models with small or even vanishing lepton mixingangle $\theta_{12}$, maximal $\theta_{23}$, zero $\theta_{13}$ and zero CPphases at the GUT scale, the currently favored LMA solution of the solarneutrino problem can be obtained in a rather natural way by RenormalizationGroup effects. We find that most of the running takes place in the energyranges above and between the see-saw scales, unless the charged lepton Yukawacouplings are large, which would correspond to a large $\tan \beta$ in theMinimal Supersymmetric Standard Model (MSSM). The Renormalization Groupevolution of the solar mixing angle $\theta_{12}$ is generically larger thanthe evolution of $\theta_{13}$ and $\theta_{23}$. A large enhancement occursfor an inverted mass hierarchy and for a regular mass hierarchy with $|m_2 -m_1| \ll |m_2 + m_1|$. We present numerical examples of the evolution of thelepton mixing angles in the Standard Model and the MSSM, in which the currentbest-fit values of the LMA mixing angles are produced with vanishing solarmixing angle $\theta_{12}$ at the GUT scale.Comment: 10 pages, 6 figures; reference added, minor changes in the text; results unchanged; final version to appear in JHE