Nuclear and Atomic Electric Dipole Moments

The permanent electric dipole moment (EDM) of a particle or system is a separation of charge along the total angular momentum and arises from interactions that directly violate parity and time reversal symmetry, and assuming CPT symmetry, violate CP. EDMs thus probe CP violating interactions in the background of the much stronger strong, weak and electromagnetic interactions. CP violation is also an essential component of Sakharov's mechanism of baryogenesis, but the Standard Model does not provide for CP violation strong enough to account for the observered baryon asymmetry. Currently, only upper limits have been set on EDMs of the neutron and of atoms and molecules, and the Standard Model contributions are many orders of magnitude smaller than these limits. This means that the experimental frontier probes new physics beyond the Standard Model, but also that experiments in at least three systems will be necessary to clarify the soures of CP violating EDMs. This review will describe the motivations and discuss current and near-term experimental endeavors to discover EDMs in a variety of systems.