Majorana Neutrinos, Neutrino Mass Spectrum, CP-Violation and Neutrinoless Double Beta-Decay: II. Mixing of Four Neutrinos

Assuming four-neutrino mixing and massive Majorana neutrinos, we study the implications of the neutrino oscillation solutions of the solar and atmospheric neutrino problems, of the results of the LSND experiment and of the constraints on neutrino oscillations, obtained in reactor and accelerator experiments, for the predictions of the effective Majorana mass in neutrinoless double beta (\betabeta-)decay, \meff. All 4-neutrino mass spectra compatible with the existing neutrino mass and oscillation data are considered. The general case of CP-nonconservation is investigated. We have also analyzed in detail the question of whether a measurement of $\meff \gtap 0.01$ eV in the next generation of \betabeta-decay experiments (NEMO3, CUORE, EXO, GENIUS), combined with the data from the solar, atmospheric, reactor and accelerator neutrino oscillation experiments and from the future neutrino mass $^3$H $\beta-$decay experiment KATRIN would allow, and under what conditions, i) to determine the absolute values of the neutrino masses and thus the neutrino mass spectrum, and ii) to establish the existence of CP-violation in the lepton sector. For certain 4-neutrino mass spectra there exists a direct relation between \meff or the lightest neutrino mass $m_1$ and the neutrino mass measured in $^3$H $\beta-$decay, $m_{\nu_e}$, and the measurement of $\meff \gtap 0.01$ eV and of $m_{\nu_e} \gtap 0.4$ eV will give the unique possibility to determine the absolute values of all four neutrino masses and to obtain information on CP-violation in the lepton sector.