Abstract
We present a stringy mechanism to generate Dirac neutrino masses by $D$-instantons in an experimentally relevant mass scale without fine-tuning. Within type IIA string theory with intersecting $D6$-branes, we spell out specific conditions for the emergence of such couplings and provide a class of supersymmetric local SU(5) grand unified models, based on the ${Z}_{2}\ifmmode\times\else\texttimes\fi{}{Z}_{2}^{\ensuremath{'}}$ orientifold compactification, where perturbatively absent Dirac neutrino masses can be generated by $D2$-brane instantons in the experimentally observed mass regime, while Majorana masses remain absent, thus providing an intriguing mechanism for the origin of small neutrino masses due to nonperturbative stringy effects.
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