Permanent Electric Dipole Moments of Atoms and Molecules

The permanent electric dipole moment (EDM) of a particle or system is a separation of charge along the direction of the total angular momentum of the system and arises from elementary particle interactions that directly violate parity and time-reversal symmetry. Assuming symmetry under combined charge-conjugation, parity, and time-reversal transformations, an EDM is a signal of simultaneous violation of charge-conjugation and parity symmetry (CP violation), and thus probes very weak interactions in the background of much stronger interactions that bind the atom and its nucleus. CP violation arises naturally in the Standard Model of strong, weak, and electromagnetic interactions through two mechanisms: the phase of a complex amplitude in the weak interactions, the CabibboKobayashiMaskawa phase, which is constrained by measurements with neutral K and B mesons, and the vacuum phase of the strong interaction, theta(QCD), which is constrained by EDM measurements. The Standard Model does not, however, appear complete. For example, the predominance of matter over antimatter in the universe is not compatible with Standard Model mechanisms of baryogenesis, and CP-violating physics beyond the Standard Model's interactions could produce the observed baryon asymmetry as well as detectable EDMs. For over five decades, experimenters have measured steadily smaller upper limits on the EDMs of atoms, molecules, and elementary particles, and the searches continue with new motivations and new techniques that promise improved sensitivity to CP violation. In this article, we set the stage for contemporary efforts and discuss current and near-term experimental endeavors to measure EDMs in a variety of systems.