---
abstract: '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.'
author:
- Paul Langacker
- 'Brent D. Nelson'
title: 'String-Inspired Triplet See-Saw from Diagonal Embedding of $SU(2)_L \subset SU(2)_A \times SU(2)_B$'
---

Introduction {#introduction .unnumbered}
============

Observations by a variety of experimental collaborations have now firmly established the hypothesis that neutrino oscillations occur and that they are the result of non-vanishing neutrino masses and mixing angles [@Maltoni:2003da; @Bahcall:2004ut; @Fogli:2005cq]. While our knowledge of neutrino mass-differences and mixings has continued to improve over recent years, there continues to be as yet no consensus on the correct mechanism for generating the quite small neutrino masses implied by the experimental data. In some respects this is similar to the case of quark masses and mixings: despite having access to even more of 