Neutron Electric Dipole Moment Experiment

A.1.a General Introduction Precision measurements of the properties of the neutron present an opportunity to search for violations of fundamental symmetries and to make critical tests of the validity of the Standard Model (SM) of electroweak (EW) interactions. These experiments have been pursued with great energy and interest since Chadwick [i] discovered the neutron in 1932. Searches for the electric dipole moment (EDM) of the neutron (nEDM) date back to a 1957 paper of Purcell and Ramsey [ii]. This paper led to an experiment using a magnetic-resonance technique at Oak Ridge National Laboratory (ORNL), where they established a value of d n = (–0.1 ± 2.4) × 10 –20 e⋅cm [iii]. In the intervening 30 years, a series of measurements of increasing precision have culminated in the current best limit of d n < 3 × 10 –26 e⋅cm (90% C.L.) obtained in measurements at the Institute Laue-Langevin (ILL) reactor at Grenoble [iv]. The physics motivation for these measurements has been widely discussed. A search for a nonzero value of the nEDM is a search for a violation of time-reversal (T) invariance. To date, there is only one measurement (a comparison of neutral K and K meson decay) in which T violation has been seen directly [v]. The asymmetry in these rates is found to be (6.6 ± 1.3 ± 1.0) × 10 –3 and is consistent with the SM. The SM prediction for the nEDM is at the 10 –31 e⋅cm level, below the reach of current measurements by six orders of magnitude [vi]. The only violation of the SM that has been observed is the recent measurements of the neutrino mass. There are many proposed models of the EW interaction that are extensions beyond the SM and that raise the predicted value of the nEDM by up to seven orders of magnitude. Some of these are already excluded by the current limit on the nEDM. The proposed experiment has the potential to reduce the acceptable range for predictions by two orders of magnitude and to provide a significant challenge to these extensions to the SM. Conversely, if a new source of T violation is present in nature, that is relevant to this hadronic system, this experiment offers an intriguing opportunity to measure a nonzero value of the nEDM. Observation of a violation of T invariance through measurement of the nEDM would be of fundamental significance. This project invokes …