Direct lattice QCD calculation of the $\theta$-induced CP-violating pion-nucleon coupling
We present the first direct lattice QCD determination of the $\theta$-induced CP-violating pion-nucleon-nucleon coupling $\tilde g_{\pi NN}$. Using overlap valence fermions on three $2+1$-flavor domain-wall ensembles at a single lattice spacing, we calculate the forward proton matrix element of the isovector pseudoscalar density in the $\theta$-vacuum to first order in $\bar\theta$. The parity-mixing effect in the external nucleon states is included in the extraction. The cluster-decomposition error-reduction method is used to improve the statistical precision. A simultaneous extrapolation in the valence- and sea-pion masses, with model averaging over 7 forms based on the Akaike information criterion, gives $\tilde g_{\pi NN}=0.0306(56)(124)\,\bar\theta$ at the physical point. The first uncertainty includes the statistical and matrix-element-fit systematic uncertainties, whereas the second reflects the spread among the extrapolation forms. Within these uncertainties, the result is consistent with the indirect determination based on the strong neutron-proton mass splitting. The present precision is limited primarily by the extrapolation from the relatively heavy sea-pion masses.