Measurement of the permanent electric dipole moment of the neutron (2001.11966v1)

Abstract: We present the result of an experiment to measure the electric dipole moment (EDM) of the neutron at the Paul Scherrer Institute using Ramsey's method of separated oscillating magnetic fields with ultracold neutrons (UCN). Our measurement stands in the long history of EDM experiments probing physics violating time reversal invariance. The salient features of this experiment were the use of a Hg-199 co-magnetometer and an array of optically pumped cesium vapor magnetometers to cancel and correct for magnetic field changes. The statistical analysis was performed on blinded datasets by two separate groups while the estimation of systematic effects profited from an unprecedented knowledge of the magnetic field. The measured value of the neutron EDM is $d_{\rm n} = (0.0\pm1.1_{\rm stat}\pm0.2_{\rm sys})\times10^{-26}e\,{\rm cm}$.

• The paper reports a precision measurement of the neutron EDM with an upper limit of |dn| < 1.8×10⁻²⁶ e·cm, narrowing constraints on CP-violating theories.
• The paper details an experimental design that employs Ramsey's method, ultracold neutrons, improved magnetic shielding, and advanced co-magnetometers to minimize systematic errors.
• The paper’s findings reinforce limits on the QCD theta parameter, thereby influencing future research on CP violation and alternative dark matter candidates.

Measurement of the Permanent Electric Dipole Moment of the Neutron

The paper under discussion conducts a meticulous examination of the permanent electric dipole moment (EDM) of the neutron, an inquiry central to understanding time-reversal symmetry violation and, by extension, CP violation within the framework of particle physics. The experiment, hosted at the Paul Scherrer Institute, employs Ramsey's method of separated oscillating magnetic fields with ultracold neutrons (UCN). The relevance of this research lies not only in testing the predictions of the Standard Model (SM) but also in probing physics beyond the SM, such as potential explanations for the observed baryon asymmetry of the universe.

Experimental Design and Methodology

The experimental setup features a co-magnetometer and an array of optically pumped cesium vapor magnetometers, instrumental in negating and accounting for fluctuations in the magnetic field. A rigorous statistical analysis was conducted on blinded datasets, managed by two independent research groups, ensuring the robustness of the findings against biases. Furthermore, a significant understanding of the magnetic field environment enhanced the accuracy of the systematic effect evaluations.

The authors employed a well-calibrated spectrometer with a detailed design described in the paper, integrating a series of refinements from previous experiments. These include improvements in the magnetic shielding and more sophisticated data acquisition techniques, allowing for a higher precision measurement of the Larmor precession frequency shift due to the neutron EDM.

Results

The measured value of the neutron EDM is provided as $$d_n = (0.0 \pm 1.1_{\rm stat} \pm 0.2_{\rm sys}) \times 10^{-26}\, e\cdot \text{cm}$$. This result does not indicate the presence of a neutron EDM beyond the experimental uncertainties, placing a stringent constraint on models predicting larger EDM values. The determined upper limit at 90% confidence level is $|d_n| < 1.8 \times 10^{-26}\, e\cdot \text{cm}$, which narrows the parameter space for theories incorporating large CP-violating phases.

Implications and Future Directions

The upper limit established by this experiment significantly bolsters the constraints on theoretical models, particularly regarding the QCD theta parameter, thus reinforcing the strong CP problem. By indirectly constraining this parameter, the research contributes valuably to the non-observation of new physics beyond the SM under current experimental capabilities.

Furthermore, this refined upper limit feeds into the landscape of dark matter research, as the null result motivates alternative investigations, like those exploring the axion as a viable dark matter candidate.

Future endeavors in the field will likely focus on further reducing the systematic uncertainties. Potential improvements include advancing the control of magnetic field gradient effects and exploring novel experimental techniques to enhance the sensitivity of neutron EDM searches. As precision increases, the results could either further constrain existing theories or unlock pathways to new understanding in fundamental physics.

This research exemplifies the critical interplay between experimental rigor and theoretical exploration in the ongoing pursuit to decode the fundamental forces shaping our universe.