Neutrino Majorana masses from string theory instanton eff ects

Finding a plausible origin for right- handed neutrino Majorana masses in semirealistic compactifications of string theory remains one of the most difficult cult problems in string phenomenology. We argue that right- handed neutrino Majorana masses are induced by non-perturbative instanton effects ects in certain classes of string compactifications in which the U(1) B-L gauge boson has a Stuckelberg mass. The induced operators are of the form e(vRvR)(-U) where U is a closed string modulus whose imaginary part transforms appropriately under B-L. This mass term may be quite large since this is not a gauge instanton and ReU is not directly related to SM gauge couplings. Thus the size of the induced right- handed neutrino masses could be a few orders of magnitude below the string scale, as phenomenologically required. It is also argued that this origin for neutrino masses would predict the existence of R-parity in SUSY versions of the SM. Finally we comment on other phenomenological applications of similar instanton effects, like the generation of a mu-term, or of Yukawa couplings forbidden in perturbation theory.