D-Instanton Generated Dirac Neutrino Masses

TL;DR

The paper tackles the challenge of obtaining tiny Dirac neutrino masses in string theory without resorting to a see-saw mechanism. It demonstrates that D2/E2-instantons in Type IIA with intersecting D6-branes can non-perturbatively generate Dirac Yukawa couplings by saturating charged fermionic zero modes, with a strength set by the exponential factor $e^{-S_{inst}}$ and without inducing Majorana terms due to the instanton zero-mode structure. An explicit local construction on a $Z_2\times Z'_2$ toroidal orientifold with specific wrapping and intersection patterns yields Dirac masses in the observed range, $m_{Dirac}\sim 2\times 10^{-3}$ eV to $0.4$ eV, for $\nu_2=1$ and $\alpha_{GUT}\sim 1/25$–$1/30$ with a Higgs VEV ~ 100 GeV. This provides a non-perturbative, string-theoretic origin for small Dirac neutrino masses without see-saw, pointing toward globally consistent Type IIA realizations and realistic phenomenology.

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 x Z'_2 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 non-perturbative stringy effects.

Figures (1)

• Figure 1: String disc diagrams that ensure the absorption of the fermionic zero modes $2\times {\bar{\lambda}}_b$ and ${\lambda_c}+{\overline{\lambda}_c}$.