\Delta  m^2_{\odot}$}}}$, $|U_{\mathrm{e} 2}|^2 = \cos^2\theta_{\odot} (1 - |U_{\mathrm{e} 1}|^2)$, $|U_{\mathrm{e} 3}|^2 = \sin^2\theta_{\odot} (1 - |U_{\mathrm{e} 1}|^2)$, and $|U_{\mathrm{e} 1}|^2 \equiv \sin^2\theta$. Thus, given , , $\theta_{\odot}$ and $\sin^2\theta$, depends, in general, on the lightest neutrino mass $m_1$, on the two Majorana CP-violating phases $\alpha_{21}$ and $\alpha_{31}$ and on the “discrete ambiguity” related to the two possible types of neutrino mass spectrum. In the case of quasi-degenerate (QD) neutrino mass spectrum, $m_1 \cong m_2 \cong m_3$, $m_1^2 \gg {\mbox{$\Delta  m^2_{\mathrm{atm}}$}},{\mbox{$ \Delta  m^2_{\odot}$}}$,  essentially does not depend on and , and the two possibilities, ${\mbox{$ \Delta  m^2_{\odot}$}}\equiv {\mbox{$ \ \Delta m^2_{21} \ $}}$ and ${\mbox{$ \Delta  m^2_{\odot}$}}\equiv {\mbox{$ \ \Delta m^2_{32} \ $}}\!\!$, lead to the same predictions for [^3] $\!\!$.

The observation of -decay will have fundamental implications for our understanding of the elementary particle interactions. It would imply, in particular, that the electron lepton charge $L_e$ and the total lepton charge $L$ are not conserved and can change by two units in the latter, and would suggest that the massive neutrinos are Majorana particles. Under the general and plausible assumptions of 3-$\nu$ mixing and massive Majorana neutrinos, -decay generated only by the (V-A) charged current weak interaction via 