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

TL;DR

This paper calculates the neutron's electric dipole moment induced by the QCD vacuum theta angle using sum rules, providing a precise estimate and constraining the theta parameter based on experimental bounds.

Contribution

It offers a next-to-next-to-leading order QCD sum rule calculation of the neutron EDM and compares it with chiral perturbation theory results, highlighting similarities and differences.

Findings

Neutron EDM d_n( heta) = 2.4 x 10^{-16} heta e cm with 40% precision

Experimental bounds imply | heta| < 3 x 10^{-10}

Comparison reveals both numerical similarity and different dominant contributions

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