Majorana neutrino mass structure induced by rigid instatons on toroidal orbifold

We study effects of D-brane instantons wrapping rigid cycles on Z2*Z2 toroidal orbifold. We compute Majorana masses induced by rigid D-brane instantons and realize bimaximal mixing matrices in certain models. We can also derive more generic mass matrices in other models. The bimaximal mixing Majorana mass matrix has a possibility to explain observed mixing angles. We also compute the mu-term matrix among more than one pairs of Higgs fields induced by rigid D-brane instantons.