To date, every measurement of the neutron electric dipole moment (EDM) has given a value of zero, but there is a good reason to believe it is actually a very small, but non zero, value.

A non-zero electric dipole moment of the neutron (or any fundamental particle) would be a violation of parity (P) and time-reversal (T) symmetry. This can be explained by the following picture: if the neutron has a finite EDM, the charge distribution is reversed under P; it is unchanged under T, but the orientation of a particle is specified by its spin, which is unchanged under P, but reverses under T. Therefore if the EDM is not zero then P and T are not conserved.

Assuming the combined operation CPT is invariant, then a measurement of T violation implies CP is also violated. This has significant consequences for cosmology. Present theories of particle physics and cosmology predict that our Universe was formed with equal parts matter and antimatter, which should by now have annihilated into radiation. To explain the dominance of matter, CP violation must exist.

CP violation has been observed in accelerator experiments studying the decays of K and B mesons, but not at a level with can explain the matter-antimatter asymmetry of the Universe. Therefore in order to explain why the Universe exists we need to study other CP violating systems such as the neutron EDM. This also provides a way to test theories of new physics, as  supersymmetry (for example) predicts a neutron EDM at a level of 10^-28-10^-26ecm, above the standard model prediction. Theories such as additional Higgs fields, and left-right symmetric models also predict a neutron EDM at a level which will be probed by new experiments.

Neutron EDM measurements have also shown that CP is conserved to a remarkable degree by the strong interaction. This would appear to require significant fine tuning of the relevant QCD parameter; but it can also be explaining by a hypothetical particle, the axion, which would effectively cancel the CP violating term in the QCD Lagrangian. Axions are also a possible dark matter candidate.

The current limit on the neutron EDM is d<2.9×10^-26ecm, set by the nEDM experiment. This was measured at the Institut Laue-Langevin using room temperature apparatus, by storing a large number of ultra-cold neutrons in a storage cell in an electric and magnetic field. The Larmor spin precession frequency was measured to a high precision for parallel and anti-parallel fields. A shift in the precession frequency between these two measurements, would be a sign of the neutron EDM.

A significant systematic error is caused by any drift in the magnetic field between measurements; as the neutron has a non-zero magnetic dipole moment, this also produces a shift in the precession frequency. The nEDM group used atomic mercury spectrometry to monitor the magnetic field to nanotesla precision.

By 1999 the experiment had reached the best sensitivity which could be achieved with the apparatus, and the collaboration started design and construction of the cryoEDM project. The Oxford group joined the project in 2003.