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A violation of CP symmetry in the very early stages of the Universe is required to explain why there is almost no antimatter in our Universe. The known (and parametrised) level of CP violation in the Standard Model cannot account for the observed baryon asymmetry, so that other sources of CP violation have been postulated in different New Physics scenarios. Most of these models provide theoretical predictions for Neutron Electric Dipole Moment (nEDM) values that are larger than 10-28 e·cm, i.e. one to two orders of magnitude below the current experimental upper limit of 3·10-26 e·cm and potentially within the reach of experiments at proposed and running ultracold neutron (UCN) sources.
The nEDM measurement with UCN are carried out using the Ramsey technique of timeseparated oscillatory fields for a precise determination of a neutron spin precession frequency in the magnetic and electric fields. The interaction of the neutron’s magnetic moment with the magnetic field (characterized by the Larmor frequency) is clearly, because of the smallness of nEDM, orders of magnitude larger than the corresponding “electric” Larmor frequency describing the interaction of a possible electric dipole moment with the electric field. Therefore, any changes of the magnetic field have to be precisely known and controlled in order to sufficiently suppress any systematic effects associated with magnetic field drifts.
The world-leading nEDM experiment located at the UCN source at the Paul Scherrer Institute is expected to improve over the best sensitivity by factor of two in the next years. At the same time the nEDM collaboration is building a new apparatus. This new project is expected to be sensitive to a nEDM at the 10-28 e·cm level. With such improved sensitivity, either the nEDM will be found, or a new limit will set tight constraints on CP-violating physics beyond the Standard Model.