Theta Vacua, QCD Sum Rules, and the Neutron Electric Dipole Moment

Abstract

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 d_n(θ) = 2.4 x 10^{-16} θe cm, to approximately 40% precision. With the current experimental bound this translates into a limit on the theta parameter of |θ| < 3 x 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.