Neutron electric dipole moment using Nf=2+1+1 twisted mass fermions

Abstract

We evaluate the neutron electric dipole moment |d→N| using lattice QCD techniques. The gauge configurations analyzed are produced by the European Twisted Mass Collaboration using Nf=2+1+1 twisted mass fermions at one value of the lattice spacing of a≃0.082 fm and a light quark mass corresponding to mπ≃373 MeV. Our approach to extract the neutron electric dipole moment is based on the calculation of the CP-odd electromagnetic form factor F3(Q2) for small values of the vacuum angle θ in the limit of zero Euclidean momentum transfer Q2. The limit Q2→0 is realized either by adopting a parametrization of the momentum dependence of F3(Q2) and performing a fit or by employing new position space methods, which involve the elimination of the kinematical momentum factor in front of F3(Q2). The computation in the presence of a CP-violating term requires the evaluation of the topological charge 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 |d→N|=0.045(6)(1)θe·fm for the ensemble with mπ=373 MeV considered. © 2016 American Physical Society.