Neutron Electric Dipole Moment Experiments

The possibility of a non-zero value for neutron EDM continues to be of central importance in physics and cosmology. A non-zero neutron EDM would be a direct evidence for time-reversal symmetry violation. It also implies CP-violation under the assumption of CPT invariance. To date, only two examples of CP-violation have been found: decays of neutral K mesons and B mesons. CP-violation is believed to have occured during the Big Bang baryogenesis that led to the present matter-antimatter asymmetry in the Universe. Although CP-violation observed in K and B meson decays can be incorporated phenomenologically within the Standard Model, the strength of the effect is not large enough to explain the matterantimatter asymmetry. It is likely that a full description of CP-violation would invoke non-standard models, many of which predict a neutron EDM large enough to be accessed experimentally1. Therefore, a sensitive measurement of the neutron EDM is of fundamental interest since it could identify new sources of CP-violation as well as physics beyond the Standard Model. In this article, we first review the history of neutron EDM experiments. The prospect for future neutron EDM experiments will then be presented. In particular, the status of proposed experiments using ultra cold neutrons at various laboratories will be discussed.