Majorana Neutrino Physics and Matter-Antimatter Asymmetry in Type IIA String Theory

We study Majorana neutrino masses and the matter-antimatter asymmetry in Type IIA string theory on a $T^6/(\mathbb{Z}_2 \times \mathbb{Z}_2)$ orientifold with intersecting D6-branes. Euclidean D2-instantons with $\text{Sp}(2)$ Chan-Paton symmetry generate both the seesaw mechanism and the Weinberg operator. The flavour vectors are not computed from a compactification but fixed by matching, so our results are an existence proof. Matching the measured mass-squared differences selects the seesaw-dominated case with normal ordering. Swampland criteria constrain the instanton action and the reheating temperature. A benchmark example reproduces the neutrino parameters, gives $η_B \approx 8.6 \times 10^{-11}$, within an order of magnitude of observation, and predicts $\langle m_{ββ} \rangle =2.594 \text{meV}$. A parameter scan shows the mass hierarchy is accommodated rather than predicted.