the relevant experimental data for an even longer period of time, no compelling model of the hierarchies of masses and mixings in the quark sector has emerged either. But most theoretical effort in the area of neutrinos goes beyond the simple Dirac-mass Yukawa operator by introducing new structures in the superpotential to account for neutrino masses, such as the see-saw mechanism (in one of its various forms, to be defined more precisely below).[^1] Thus neutrinos are likely to be very special in the Standard Model – and its supersymmetric extensions – and may thereby provide a unique window into high-scale theories that the quark sector fails to illuminate.

It has thus far been mostly in vain that we might look to string theory for some guidance in how to approach the issue of flavor. In part this is because of the vast number of possible vacua in any particular construction, each with its own set of fields and superpotential couplings between them. On the other hand the problem of generating small neutrino masses may be one of the most powerful discriminants in finding realistic constructions. This was one of the conclusions of a recently completed survey [@Giedt:2005vx] of a large 