Measurement of the Permanent Electric Dipole Moment of the Neutron

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. 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 ${}^{199}\mathrm{Hg}$ comagnetometer 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 $d_n=(0.0\pm {1.1}_{\mathrm{stat}}\pm {0.2}_{\mathrm{sys}})\times {10}^{-26}e.\mathrm{cm}$.

Figure 1

Scheme of the spectrometer used to search for an nEDM. A nonzero signal manifests as a shift of the magnetic resonance frequency of polarized UCNs in a magnetic field $B_0$ when exposed to an electric field of strength $E$.

Figure 2

Illustration of the fit to the Ramsey central fringe. Data without an electric field are omitted. The data scatter around the four working points. Faded data and lines are for the blinded case (illustration for a very large artificial EDM).

Figure 3

A typical sequence of cycles. The upper plot shows the neutron frequency $f_n$ as a function of the cycle number; the lower plot shows the frequency ratio $\mathcal{R}$. The colors correspond to the high-voltage polarity (blue, negative; red, positive; black, zero). The vertical lines separate the subsequences.

Figure 4

Crossing point analysis: The corrected electric dipole moment $d_n^{\mathrm{corr}}$ is plotted vs ${\mathcal{R}}^{\mathrm{corr}}$ (see the text for the exact definition of $d_n^{\mathrm{corr}}$ and ${\mathcal{R}}^{\mathrm{corr}}$). Upward-pointing (red) and downward-pointing (blue) triangles represent sequences in which $B_0$ was pointing upward and downward, respectively. The fitted value of ${\mathcal{R}}_{\times }$ is represented by the green vertical band ($1\sigma$), and the vertical dashed line represents the ratio of gyromagnetic ratios calculated from the literature values of ${\gamma }_n$ [31] and ${\gamma }_{\text{Hg}}$ [32]. The lower panel shows the normalized fit residuals.