Pseudo-Dirac neutrinos as a potential complete solution to the neutrino oscillation puzzle

A solution for the neutrino mass and mixing pattern is proposed which is compatible with all available experimental data on neutrino oscillations. This solution involves Majorana neutrinos of the pseudo-Dirac type, i.e. $m_{\rm Majorana} \ll m_{\rm Dirac}$. The solar and atmospheric neutrino observations are mainly explained as $\nu_e - \nu_e^S$ and $\nu_\mu - \nu_\mu^S$ oscillations, w here S indicates the sterile (``righthanded'') partner of each neutrino generation, while the LSND result is interpreted in terms of standard $\nu_\mu - \nu_e$ oscillations. The resulting constrai nts on $\nu_\mu - \nu_\tau$ and $\nu_\tau - \nu_\tau^S$ oscillations are also discussed. This solution leaves room for a hierarchical mass and mixing scheme with a $\nu_\tau$ mass in the few eV r ange, as favoured by some dark matter scenarios. The apparent conflict with standard Big Bang nucleosynthesis is addressed and the implications for current and future experiments are discussed. I t is argued that both short and long baseline accelerator neutrino experiments are needed in order to decide between this solution and other oscillation scenarios.