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 60 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 θ¯ 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.

Electric dipole moments of atoms, molecules, nuclei, and particles

A description is presented of apparatus used to carry out an experimental search for an electric dipole moment of the neutron, at the Institut Laue-Langevin (ILL), Grenoble. The experiment incorporated a cohabiting atomic-mercury magnetometer in order to reduce spurious signals from magnetic field fluctuations. The result has been published in an earlier letter [1]; here, the methods and equipment used are discussed in detail.

/pdf/apparatus-for-measurement-of-the-electric-dipole-moment-of-2xmo3w6ko5.pdf

Apparatus for measurement of the electric dipole moment of the neutron using a cohabiting atomic-mercury magnetometer

Nuclear spin maser with an artificial feedback mechanism

We report results of a new technique to measure the electric dipole moment of $^{129}\mathrm{Xe}$ with $^{3}\mathrm{He}$ comagnetometry. Both species are polarized using spin-exchange optical pumping, transferred to a measurement cell, and transported into a magnetically shielded room, where SQUID magnetometers detect free precession in applied electric and magnetic fields. The result from a one week measurement campaign in 2017 and a 2.5 week campaign in 2018, combined with detailed study of systematic effects, is ${d}_{A}(^{129}\mathrm{Xe})=(1.4\ifmmode\pm\else\textpm\fi{}6.{6}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}2.{0}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}28}\text{ }\text{ }e\text{ }\mathrm{cm}$. This corresponds to an upper limit of $|{d}_{A}(^{129}\mathrm{Xe})|l1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}27}\text{ }\text{ }e\text{ }\mathrm{cm}$ (95% C.L.), a factor of 5 more sensitive than the limit set in 2001.

/pdf/new-limit-on-the-permanent-electric-dipole-moment-of-xe-129-bhk93fynxy.pdf

New Limit on the Permanent Electric Dipole Moment of Xe 129 Using He 3 Comagnetometry and SQUID Detection

We describe a 3He
magnetometer capable to measure high magnetic fields (B> 0.1 T) with a
relative accuracy of better than 10-12. Our approach is based on the measurement of the free
induction decay of gaseous, nuclear spin polarized 3He following a resonant radio
frequency pulse excitation. The measurement sensitivity can be attributed to the long
coherent spin precession time T2
 ∗ being of order minutes which is achieved for spherical
sample cells in the regime of “motional narrowing” where the disturbing influence of field
inhomogeneities is strongly suppressed. The 3He gas is spin polarized in situ using a new,
non-standard variant of the metastability exchange optical pumping. We show that
miniaturization helps to increase T2
 ∗ further and that the measurement sensitivity is not
significantly affected by temporal field fluctuations of order 10-4.

Ultrasensitive 3He magnetometer for measurements of high magnetic fields

Consideration is given to frequency estimates for continuous data trains of a fixed length amounting to at least five oscillations of a sinusoidal signal of constant frequency accompanied by low-level noise. The minimum variance of the frequency estimation is derived in terms of the noise spectral density at frequencies close to that of the signal. The extent to which several practical methods of frequency measurement can reach this limit is analysed. The case in which the frequency of the signal source may vary slightly within each data train is also included.

K F Smith

Papers

Minimum variance of frequency estimations for a sinusoidal signal with low noise