Theta vacua, QCD sum rules, and the neutron electric dipole moment Original Research Article

We present a detailed study of the electric dipole moment of the neutron induced by a vacuum theta angle within the framework of QCD sum rules. At next-to-next-to-leading order in the operator product expansion, we find the result dn(θ)=1.2×10−16θ̄e·cm, to approximately 40% precision. With the current experimental bound this translates into a limit on the theta parameter of |θ̄|<6×10−10. We compare this result with the long-standing estimates obtained within chiral perturbation theory, and observe a numerical similarity, but also significant differences in the source of the dominant contribution.