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In SO(10) grand unified theories (GUTs) the hierarchy which is present in theDirac mass term of the neutrinos is generically as strong as the one in theup-type quark mass term. We propose a mechanism to partially or completelycancel this hierarchy in the light neutrino mass matrix in the seesaw context.The two main ingredients of the cancellation mechanism are the existence ofthree fermionic gauge singlets and of a discrete flavor symmetry G_f which isbroken at a higher scale than SO(10). Two realizations of the cancellationmechanism are presented. The realization based on the Frobenius group T7 = Z7 xZ3 leads to a partial cancellation of the hierarchy and relates maximal 2-3lepton mixing with the geometric hierarchy of the up-quark masses. In therealization with the group Sigma(81) the cancellation is complete andtri-bimaximal lepton mixing is reproduced at the lowest order. In both cases,to fully accommodate the leptonic data we take into account additional effectssuch as effects of higher-dimensional operators involving more than one flavon.The heavy neutral fermion mass spectra are considered. For both realizations weanalyze the flavon potential at the renormalizable level as well as ways togenerate the Cabibbo angle.

Lepton Mixing and Cancellation of the Dirac Mass Hierarchy in SO(10) GUTs with Flavor Symmetries T7 and Sigma(81)