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

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)(theta) = 1.2 X 10(-16)<(theta)over bar>e(.)cm, to approximately 40% precision. With the current experimental bound this translates into a limit on the theta parameter of \<(theta)over bar>\ < 6 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. (C) 2000 Elsevier Science B.V. All rights reserved.