Strong CP Problem in Type IIA Toroidal Orientifold String Theory

Abstract

We present a globally consistent Type IIA $T^6/(\mathbb{Z}_2 \times \mathbb{Z}_2)$ orientifold with intersecting D6-branes that dynamically solves the strong CP problem via the four-form flux mechanism. The construction satisfies all RR tadpole, K-theory, and SUSY conditions, stabilizes all moduli, and uplifts to dS vacuum with an anti-D6-brane. Four new results beyond our earlier proof-of-principle \cite{Liu:2025tnx} are given: (i) an explicit one-to-one dictionary between field-theory four-form parameters and string fluxes, including flux quantization constraints; (ii) a quantitative anti-D6 backreaction estimate showing $|\delta \bar{\theta}|<10^{-11}$; (iii) an analysis of swampland constraints that are simultaneously satisfied only in a parameter window; (iv) a comparison with Type IIB LVS and heterotic solutions. The predicted axion decay constant $f_a$ and mass $m_a$ are within reach of next generation observations.