Neutron Electric Dipole Moment (nEDM) Research Project @ Oak Ridge
The possible existence of a non-zero electric dipole moment of the neutron is of great fundamental interest and directly impacts our understanding of the nature of electro-weak and strong interactions. The experimental search for this moment has the potential to reveal new sources of T and CP violation and to challenge calculations that propose extensions to the Standard Model. In addition, the small value for the neutron EDM continues to raise the issue of why the strength of the CP-violating terms in the QCD Lagrangian are so small. This seems to suggest the existence of a new fundamental symmetry that blocks strong CP-violating processes.
Neutron Electric Dipole Moment (nEDM) Research Project @ LANL
The permanent Electric Dipole Moment (EDM) is a probe for the violation of time reversal (T) symmetry, and thus of the combined symmetries of charge conjugation and parity inversion (CP), assuming CPT invariance. It has been identified as one of the highest priority research projects in the US Nuclear Physics Long-Range Plan, both in the years 2010 and 2015. The EDM can test the premise of fundamental symmetries in modern quantum field theories---with high sensitivity---because EDM is largely immune to the backgrounds and uncertainties from known physics. Predictions of the EDM arising from the standard model processes, such as the CP-violating complex phase of the CKM matrix, are many orders of magnitude smaller than current experimental limits.
Last year, the PSI group published a new nEDM limit of (0.0 ± 1.1stat ± 0.2sys)\times 10-26 e cm [Abel2020]. This result includes a moderate statistical improvement and a factor of 5 reduction in systematic errors. The goal of the LANL project is to take advantage of the recently upgraded UCN source in area B at LANSCE to get competitive limits on nEDM over the next 5 years.
An order of magnitude increase in the LANL UCN source intensity, and no other technological advances, would make possible an nEDM search at the 10-27e-cm level of sensitivity [Ito2018]. This initiative is distinct from the SNS nEDM experiment, which features superfluid helium as the UCN source, the detector for neutron spin precession, and the insulating medium to sustain a high electric field. The LANL nEDM experiment operates in vacuum at room temperature and uses the method of Ramsey's separated oscillatory fields (developed and perfected in prior nEDM experiments). This effort complements the SNS nEDM experiment, serving as risk mitigation if unanticipated technical challenges or further delays are incurred by the latter project. This mid-scale effort at LANL will provide immediate training (and dissertation opportunities) in precision EDM measurements to students and postdocs, who will support the SNS nEDM experiment as that project comes online.
The nEDM@LANL experiment uses a double-cell geometry; neutrons from the two precession cells are subject to a common magnetic field (~1 μT) and an electric field (~1 MV/m) pointing in opposite directions. The difference in the precession frequencies should be less susceptible to field drifts. In addition, laser-polarized Hg co-magnetometers will be used to independently monitor the drift of the magnetic fields experienced by UCN. A larger UCN density in the nEDM measurement cell increases the statistical power to detect an EDM. To this end, the main thrust of the LANL nEDM experiment is on the source intensity upgrade.
Pushing the nEDM sensitivity below 10-26 e-cm will require further work to achieve exquisite control and low-noise measurement of the magnetic fields. Over the next few years, the research focus is on pushing the state-of-the-art techniques of magnetometry. Using the completed MSE, we are gaining experience with the mercury magnetometer, the Cs+3He hybrid atomic magnetometer, and fluxgate magnetometers at low fields. To further tailor the field profile, we are developing novel coils that can produce a series of gradients that form a complete orthonormal basis set, described by spherical harmonic functions, from which any configuration can be produced. The goal is to use these coils to actively tune and shape the gradients by controlling the supply currents with feedback from measurements from an array of magnetometers. We plan to dedicate the next few years to implementing the necessary field controls and taking physics data using this EDM apparatus with the UCN source at LANSCE.