Revisiting pseudo-Dirac neutrino scenario after recent solar neutrino data

Abstract

It is still unknown whether the mass terms for neutrinos are of Majorana type or of Dirac type. An interesting possibility, known as pseudo-Dirac scheme, combines these two with a dominant Dirac mass term and a subdominant Majorana one. As a result, the mass eigenstates come in pairs with a maximal mixing and a small splitting determined by the Majorana mass. This will affect the neutrino oscillation pattern for long baselines. We revisit this scenario employing recent solar neutrino data, including the seasonal variation of the $^{7}\mathrm{Be}$ flux recently reported by BOREXINO. We constrain the splitting using these data and find that both the time integrated solar neutrino data and the seasonal variation independently point towards a new pseudo-Dirac solution with nonzero splitting for ${\ensuremath{\nu}}_{2}$ of $\mathrm{\ensuremath{\Delta}}{m}_{2}^{2}\ensuremath{\simeq}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{eV}}^{2}$. We propose alternative methods to test this new solution. In particular, we point out the importance of measuring the solar neutrino flux at the intermediate energies $1.5\text{ }\text{ }\mathrm{MeV}<{E}_{\ensuremath{\nu}}<3.5\text{ }\text{ }\mathrm{MeV}$ (below the Super-Kamiokande detection threshold) as well as a more precise measurement of the $pep$ flux.