Electric Dipole Moments of the Atoms, Molecules, Nuclei and Particles

TL;DR

This paper reviews the significance, theoretical background, and experimental efforts related to electric dipole moments (EDMs) of particles, nuclei, atoms, and molecules, highlighting their role in understanding CP violation and new physics.

Contribution

It provides a comprehensive overview of the theoretical motivations, experimental techniques, and recent developments in EDM research, connecting fundamental physics to observable phenomena.

Findings

EDMs serve as signals of CP violation and T-violation.

Current experiments set upper limits on EDMs, constraining new physics.

Theoretical models link EDM measurements to beyond-Standard-Model physics.

Abstract

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, and theory at several scales is crucial to interpret these limits. Nuclear theory provides connections from Standard-Model and Beyond-Standard-Model physics to the observable EDMs, and atomic and molecular theory reveal how CP-violation is manifest in these systems. EDM results in hadronic systems require that the Standard Model QCD parameter of $\bar\theta$ must be exceptionally small, which could be explained by the existence of axions - also a candidate dark-matter particle. Theoretical results on electroweak baryogenesis show that new physics is needed to explain the dominance of matter in the universe. 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.