Neutron electric dipole moment using N-f=2+1+1 twisted mass fermions

摘要

We evaluate the neutron electric dipole moment vertical bar(d) over right arrow (N)vertical bar 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 similar or equal to 0.082 fm and a light quark mass corresponding to m(pi) similar or equal to 373 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) -> 0 is realized either by adopting a parametrization 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 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 vertical bar(d) over right arrow (N)vertical bar = 0.045(6)(1)(theta) over bare . fm for the ensemble with m(pi) = 373 MeV considered.