String-inspired triplet seesaw mechanism from diagonal embedding ofSU(2)L⊂SU(2)A×SU(2)B

Motivated by string constructions, we consider a variant on the Type II see-saw mechanism involving the exchange of triplet representations of $SU(2{)}_L$ in which this group arises from a diagonal embedding into $SU(2{)}_A\times SU(2{)}_B$. A natural assignment of standard model lepton doublets to the two underlying gauge groups results in a bimaximal pattern of neutrino mixings and an inverted hierarchy in masses. Simple perturbations around this leading-order structure can accommodate the observed pattern of neutrino masses and mixings.