A fresh look into the neutron EDM and magnetic susceptibility

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 ν. 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 $|\theta |⩽2\times {10}^{\mathrm{-9}}$ for the strong CP-violating angle is obtained from QSSR, while the strongest upper bound $|\theta |⩽1.3\times {10}^{\mathrm{-10}}$ 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.