Neutron electric dipole moment using $N_f{=}2{+}1{+}1$ twisted mass fermions

We evaluate the neutron electric dipole moment $vert vec{d}_Nvert$ using lattice QCD techniques. The gauge configurations analyzed are produced by the European Twisted Mass Collaboration using $N_f{=}2{+}1{+}1$ twisted mass fermions at one value of the lattice spacing of $a simeq 0.082 {rm fm}$ and a light quark mass corresponding to $m_{pi} simeq 373 {rm MeV}$. Our approach to extract the neutron electric dipole moment is based on the calculation of the $CP$-odd electromagnetic form factor $F_3(Q^2)$ for small values of the vacuum angle $theta$ in the limit of zero Euclidean momentum transfer $Q^2$. The limit $Q^2 to 0$ is realized either by adopting a parameterization of the momentum dependence of $F_3(Q^2)$ and performing a fit, or by employing new position space methods, which involve the elimination of the kinematical momentum factor in front of $F_3(Q^2)$. The computation in the presence of a $CP$-violating term requires the evaluation of the topological charge ${cal Q}$. This is computed by applying the cooling technique and the gradient flow with three different actions, namely the Wilson, the Symanzik tree-level improved and the Iwasaki action. We demonstrate that cooling and gradient flow give equivalent results for the neutron electric dipole moment. Our analysis yields a value of $vert vec{d}_Nvert=0.045(6)(1) bar{theta} e cdot {rm fm}$ for the ensemble with $m_pi=373$ MeV considered.