HOW CAN WE OBTAIN A LARGE MAJORANA-MASS IN CALABI-YAU MODELS

In a certain type of Calabi-Yau superstring models it is clarified tha t the symmetry breaking occurs by stages at two large intermediate ene rgy scales and that two large intermediate scales induce large Majoran a-masses of right-handed neutrinos. Peculiar structure of the effectiv e nonrenormalizable interactions is crucial in the models. In this sch eme Majorana-masses possibly amount to O(10(9 approximately 10) GeV) a nd see-saw mechanism is at work for neutrinos. Based on this scheme we propose a viable model which explains the smallness of masses for thr ee kinds of neutrino nu(e), nu(mu) and nu(tau). Special forms of the n onrenormalizable interactions can be understood as a consequence of an appropriate discrete symmetry of the compactified manifold.