We obtain a $3+2$ neutrino spectrum within a left-right symmetric framework by invoking a singular double seesaw mechanism. Higgs doublets are employed to break $S{U}_R(2)$ and three additional fermions, singlets under the left-right symmetric gauge group, are included. The introduction of a singularity into the singlet fermion Majorana mass matrix results in a light neutrino sector of three neutrinos containing predominantly ${\nu }_{\alpha L}$, $\alpha =e,\mu ,\tau$, separated from two neutrinos containing a small ${\nu }_{\alpha L}$ component. The resulting active-sterile mixing in the $5\times 5$ mixing matrix is specified once the mass eigenvalues and the $3\times 3$ submatrix corresponding to the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix are known.

• Received 23 May 2004

DOI:https://doi.org/10.1103/PhysRevD.70.053012