A Revised Experimental Upper Limit on the Electric Dipole Moment of the Neutron

Abstract: We present for the first time a detailed and comprehensive analysis of the experimental results that set the current world sensitivity limit on the magnitude of the electric dipole moment (EDM) of the neutron. We have extended and enhanced our earlier analysis to include recent developments in the understanding of the effects of gravity in depolarizing ultracold neutrons (UCN); an improved calculation of the spectrum of the neutrons; and conservative estimates of other possible systematic errors, which are also shown to be consistent with more recent measurements undertaken with the apparatus. We obtain a net result of $d_\mathrm{n} = -0.21 \pm 1.82 \times10^{-26}$ $e$cm, which may be interpreted as a slightly revised upper limit on the magnitude of the EDM of $3.0 \times10^{-26}$ $e$cm (90% CL) or $ 3.6 \times10^{-26}$ $e$cm (95% CL). This paper is dedicated by the remaining authors to the memory of Prof. J. Michael Pendlebury.

• The paper refines the neutron’s EDM limit by integrating updated ILL data with modern analytical techniques to tighten CP violation constraints.
• It employs advanced methodologies like gravity-enhanced depolarization corrections and a global fitting procedure to minimize systematic errors.
• The findings guide future experimental designs at facilities like PSI, paving the way for probes into new physics beyond the Standard Model.

An Overview of "A Revised Experimental Upper Limit on the Electric Dipole Moment of the Neutron"

This paper presents a rigorous re-examination of the experimental constraints on the electric dipole moment (EDM) of the neutron. The comprehensive analysis integrates both the original dataset and contemporary theoretical advancements, providing a refined assessment of the limitations on the neutron EDM, a critical parameter in probing CP violation and potential physics beyond the Standard Model.

Methodological Refinements and Analysis

The study utilizes an extensive dataset collected in earlier experiments conducted at the Institut Laue-Langevin (ILL) between 1998 and 2002. The authors have enriched this data set with updated computational techniques and a nuanced understanding of systematic effects.

Key improvements in the analysis include:

1. Gravity-Enhanced Depolarization: The authors address the implications of gravitational effects on ultracold neutrons (UCN). This involved an enhanced calculation of the neutron spectrum and consideration of spectrum-dependent depolarization effects.
2. Systematic Error Evaluation: The impact of various systematic errors was meticulously evaluated, including detailed modeling of potential false EDM signals attributed to magnetic-field gradients, mechanical vibrations, and environmental magnetic dipoles.
3. Globally Optimal Fitting Procedure: A sophisticated global fitting strategy was implemented, synthesizing auxiliary measurement data to effectively model and mitigate systematic biases. This involved fitting parameters such as dipole moments and magnetic field gradients.

Results and Numerical Findings

The refined analysis yields an upper limit on the neutron EDM of $d_\mathrm{n} = \finalEdm \pm \finalUncert \times10^{-26}$ \ecm. The experimental results can be interpreted as setting the revised upper bounds at the 90% and 95% confidence levels, detailed as $\limitNinety \times10^{-26}$ \ecm\ and $\limitNinetyfive \times10^{-26}$ \ecm, respectively.

Theoretical and Practical Implications

The presented constraints on the neutron EDM have significant implications for theoretical models that attempt to explain CP violation. As EDM measurements are a sensitive approach to test the Standard Model and search for new physics, narrowing the upper limits on the neutron EDM offers critical guidance for theoretical advancements in particle physics.

The paper also provides useful insight for the design and implementation of future experiments aimed at measuring the neutron EDM. By addressing complex systematic effects and implementing advanced analytical methods, this work sets a methodological precedent for further experimental endeavors in the field.

Future Prospects

The apparatus and analytical improvements elucidated in this paper are poised for continued utilization and enhancement. The authors note the device's current operation at the Paul Scherrer Institute (PSI), leveraging the latest technological upgrades to push the sensitivity envelope even further.

Overall, this work consolidates former findings with modern theoretical insights, offering a refined view of the neutron EDM landscape that will inform ongoing theoretical and experimental research pursuits.