A Fresh Look into the Neutron EDM and Magnetic Susceptibility

TL;DR

This paper critically reexamines the estimates of the neutron electric dipole moment using chiral and QCD spectral sum rules, highlighting methodological issues and refining bounds on CP violation parameters.

Contribution

It provides a detailed analysis of the impact of different interpolating currents and scale choices on NEDM estimates, improving the accuracy of CP violation bounds.

Findings

QSSR results overestimate NEDM by a factor of 4 when using non-optimal operators

The strongest CP violation bound is |θ|<1.3×10^-10 from the chiral approach at scale M_N

Proton magnetic susceptibility is re-estimated, impacting NEDM calculations

Abstract

We reexamine the estimate of the neutron Electric Dipole Moment (NEDM) from chiral and QCD spectral sum rules (QSSR) approaches. In the former, we evaluate the pion mass corrections which are about 5% of the leading Log. results. However, the chiral estimate can be affected by the unknown value of the renormalizaton scale nu. For QSSR, we analyze the effect of the nucleon interpolating currents on the existing predictions. We conclude that previous QSSR results are not obtained within the optimal choice of these operators, which lead to an overestimate of these results by about a factor 4. The weakest upper bound |theta|< 2 10^-9 for the strong CP-violating angle is obtained from QSSR, while the strongest upper bound |theta|< 1.3 10^-10 comes from the chiral approach evaluated at the scale \nu=M_N. We also re-estimate the proton magnetic susceptibility, which is an important input in the QSSR estimate of the NEDM.