Computation of D-brane instanton induced superpotential couplings - Majorana masses from string theory

We perform a detailed conformal field theory analysis of D2-brane instanton effects in four-dimensional type IIA string vacua with intersecting D6-branes. In particular, we explicitly compute instanton induced fermion two-point couplings which play the role of perturbatively forbidden Majorana mass terms for right-handed neutrinos or MSSM mu-terms. These results can readily be extended to higher-dimensional operators. In concrete realizations of such non-perturbative effects, the Euclidean D2-brane has to wrap a rigid, supersymmetric cycle with strong constraints on the zero mode structure. Their implications for Type IIA compactifications on the T^6/(Z_2 x Z_2) orientifold with discrete torsion are analyzed. We also construct a local supersymmetric GUT-like model allowing for a class of Euclidean D2-branes whose fermionic zero modes meet all the constraints for generating Majorana masses in the phenomenologically allowed regime. Together with perturbatively realized Dirac masses, these non-perturbative couplings give rise to the see-saw mechanism.