Geometric Resolution of the Strong CP Problem via Dynamical Relaxation in the Zhang Effective Metric

The strong CP problem — the extreme smallness of the QCD CP-violating θ-angle constrained by neutron electric dipole moment measurements — is addressed within the rotational-order geometric unification framework. The internal S³ orientation field of the Zhang Effective Metric implements the Peccei-Quinn axion mechanism geometrically, without introducing a new fundamental scalar particle or a standalone global U(1) PQ symmetry. Through 15D Kaluza-Klein reduction, this fibre zero mode couples to the QCD topological term via mixed Chern–Simons interaction. At high temperature, QCD instanton effects are exponentially suppressed and the field is free; as the universe cools below the QCD scale, the instanton-generated periodic potential activates. Hubble friction damps the field’s motion, relaxing the effective θ-angle toward the CP-conserving vacuum. This geometric axion also serves as an ultralight dark matter candidate, accessible to experimental searches including ADMX and CASPEr.