Connecting bimaximal neutrino mixing to a light sterile neutrino

It is shown that if small neutrino masses owe their origin to the conventional seesaw mechanism and the Maki-Nakagawa-Sakata (MNS) mixing matrix is in the exact bimaximal form, then there exist symmetries in the theory that allow one of the right-handed neutrinos to become naturally massless, making it a candidate for the sterile neutrino discussed in the literature. Departures from the exact bimaximal limit lead to a tiny mass for the sterile neutrino as well as its mixing to the active neutrinos. This provides a minimal theoretical framework where a simultaneous explanation of the solar, atmospheric, and Liquid Scintillation Neutrino Detector (LSND) observations within the so-called $3+1$ scenario may be possible.