New Type of Seesaw Mechanism for Neutrino Masses

In a wide class of unified models there is an additional (and possibly dominant) term in the neutrino mass formula that under the simplest assumption takes the form $M_{\nu }=(M_N+M_N^T)u/M_G$, where $M_N$ is the neutrino Dirac mass matrix, and $u=O(M_W)$. This makes possible highly predictive models. A generalization of this form yields realistic neutrino masses and mixings more readily than the usual seesaw formula in some models. The conditions for resonant enhancement of leptogenesis can occur naturally in such models.