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

J. M. Pendlebury, S. Afach, N. J. Ayres, C. A. Baker, G. Ban, G. Bison, K. Bodek, M. Burghoff, P. Geltenbort, K. Green, W. C. Griffith, M. van der Grinten, Z. D. Grujic, P. G. Harris, V. Helaine, P. Iaydjiev, S. N. Ivanov, M. Kasprzak, Y. Kermaidic, K. Kirch, H. -C. Koch, S. Komposch, A. Kozela, J. Krempel, B. Lauss, T. Lefort, Y. Lemiere, D. J. R. May, M. Musgrave, O. Naviliat-Cuncic, F. M. Piegsa, G. Pignol, P. N. Prashanth, G. Quemener, M. Rawlik, D. Rebreyend, J. D. Richardson, D. Ries, S. Roccia, D. Rozpedzik, A. Schnabel, P. Schmidt-Wellenburg, N. Severijns, D. Shiers, J. A. Thorne, A. Weis, O. J. Winston, E. Wursten, J. Zejma, G. Zsigmond

TL;DR

This work reanalyzes the ILL neutron EDM data (1998–2002) with enhanced modeling of gravity-driven depolarization, refined ultracold neutron spectra, and expanded systematic checks. The authors employ Ramsey spectroscopy with a mercury co-magnetometer, perform a global fit that accounts for dipole and quadrupole magnetic-field components, Earth rotation, and updated γ-ratio values, and apply targeted auxiliary measurements to validate field corrections. The resulting neutron EDM is consistent with zero within improved uncertainties, yielding a final limit of |d_n| < 3.0 × 10^-26 e cm (90% CL) and |d_n| < 3.6 × 10^-26 e cm (95% CL). This work tightens constraints on CP-violating physics beyond the Standard Model and informs ongoing high-sensitivity nEDM experiments, including upgrades at PSI and related techniques such as spin-echo spectrum analytics.

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.

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

TL;DR

This work reanalyzes the ILL neutron EDM data (1998–2002) with enhanced modeling of gravity-driven depolarization, refined ultracold neutron spectra, and expanded systematic checks. The authors employ Ramsey spectroscopy with a mercury co-magnetometer, perform a global fit that accounts for dipole and quadrupole magnetic-field components, Earth rotation, and updated γ-ratio values, and apply targeted auxiliary measurements to validate field corrections. The resulting neutron EDM is consistent with zero within improved uncertainties, yielding a final limit of |d_n| < 3.0 × 10^-26 e cm (90% CL) and |d_n| < 3.6 × 10^-26 e cm (95% CL). This work tightens constraints on CP-violating physics beyond the Standard Model and informs ongoing high-sensitivity nEDM experiments, including upgrades at PSI and related techniques such as spin-echo spectrum analytics.

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 cm, which may be interpreted as a slightly revised upper limit on the magnitude of the EDM of cm (90% CL) or cm (95% CL). This paper is dedicated by the remaining authors to the memory of Prof. J. Michael Pendlebury.

Paper Structure

This paper contains 49 sections, 41 equations, 19 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Neutron frequency determination
  3. Applied cuts
  4. Manual cuts
  5. Sub-runs
  6. Mercury $\chi^2/\nu$ cut
  7. Mercury frequency uncertainty
  8. Magnetic-field jumps
  9. First-cycle cut
  10. Ramsey residuals outlier cut
  11. High voltage
  12. Frequency ratio
  13. False EDM signals
  14. Introduction
  15. False-EDM analysis: First iteration
  16. ...and 34 more sections

Figures (19)

  • Figure 1: (Color online) From Baker_2006. Measured EDM (binned data) as a function of the relative frequency shift of neutrons and Hg. The solid red line is a linear best fit.
  • Figure 2: (Color online) From Baker_2014. Spin-up and spin-down neutron counts for a single run fitted to the Ramsey curve (Eq. \ref{['eqn: Ramsey with phase']}).
  • Figure 3: (Color online) From Baker_2014. The distribution of $\chi ^2/\nu$ from mercury frequency fits. See text for details.
  • Figure 4: (Color online) The distribution of uncertainties from mercury frequency fits. See text for details.
  • Figure 5: (Color online) From Baker_2014. Distribution of stretch values from the fits to the Ramsey curve
  • ...and 14 more figures