Anomalous electric dipole moment
In particle physics, the anomalous electric dipole moment, or the electric dipole moment of a particle in short, is the electric dipole moment of a particle. There is a symmetry, the CP symmetry, which if exact and unbroken will predict an exactly zero electric dipole moment for particles. However, we know at least in the Yukawa sector from neutral kaon oscillations that CP is broken. Experiments have been performed to measure the electric dipole moment of various particles like the electron and the neutron. Many models beyond the standard model with additional CP-violating terms generically predict a nonzero electric dipole moment and are hence sensitive to such new physics. Instanton corrections from a nonzero θ term in quantum chromodynamics predict a nonzero electric dipole moment for the neutron (it is easier to measure the electric dipole moment in a neutral particle) which have not been observed. This is the strong CP problem and is a prediction of chiral perturbation theory.