This paper has been seven years in the making and counting. I have only participated in the last two and half years of this massive undertaking and found the process to be profoundly humbling. I am tremendously relieved to see this posted online and am looking forward to all of the feedback from the community. .
A permanent electric dipole moment (EDM) of a particle or system is a separation of charge along its angular-momentum axis and is a direct signal of T-violation and, assuming CPT symmetry, CP violation. For over sixty years EDMs have been studied, first as a signal of a parity-symmetry violation and then as a signal of CP violation that would clarify its role in nature and in theory. Contemporary motivations include the role that CP violation plays in explaining the cosmological matter-antimatter asymmetry and the search for new physics. Experiments on a variety of systems have become ever-more sensitive, but provide only upper limits on EDMs. Theory at several levels is crucial: elementary-particle phenomonolgy connects the Standard Model and the theta parameter in QCD that must be exceptionally small, which could be explained by the existence of axions, which may also be the dark-matter particles. Connections to electroweak baryogenesis conclude that new physics is needed to explain the dominance of matter in the universe; nuclear theory provides connections of Standard-Model and Beyond-Standard-Model physics to the observable EDMs, and atomic and molecular theory reveal how CP-violation is manifest at that scale. Experimental and theoretical efforts continue to expand with new ideas and new questions, and this review provides a broad overview of theoretical motivations and interpretations as well as details about experimental techniques, experiments, and prospects. The intent is to provide specifics and context as this exciting field moves forward.
Submitted to Review of Modern Physics