Neutrino Majorana Masses from String Theory Instanton Effects

TL;DR

This paper proposes that non-perturbative string theory instanton effects can generate Majorana masses for right-handed neutrinos, potentially explaining their small observed masses and predicting R-parity in supersymmetric models.

Contribution

It introduces a novel mechanism where string instantons induce Majorana neutrino masses, linking them to moduli fields and providing a phenomenologically viable mass scale.

Findings

Instanton effects can generate right-handed neutrino masses below the string scale.

The mechanism predicts the existence of R-parity in SUSY models.

Similar instanton effects could generate other forbidden Yukawa couplings.

Abstract

Finding a plausible origin for right-handed neutrino Majorana masses in semirealistic compactifications of string theory remains one of the most difficult problems in string phenomenology. We argue that right-handed neutrino Majorana masses are induced by non-perturbative instanton effects in certain classes of string compactifications in which the $U(1)_{B-L}$ gauge boson has a St\"uckelberg mass. The induced operators are of the form $e^{-U}\nu_R\nu_R$ 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 $Re U$ 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.