Reduced Limit on the Permanent Electric Dipole Moment of ${}^{199}\mathrm{Hg}$

This Letter describes the results of the most recent measurement of the permanent electric dipole moment (EDM) of neutral ${}^{199}\mathrm{Hg}$ atoms. Fused silica vapor cells containing enriched ${}^{199}\mathrm{Hg}$ are arranged in a stack in a common magnetic field. Optical pumping is used to spin polarize the atoms orthogonal to the applied magnetic field, and the Faraday rotation of near-resonant light is observed to determine an electric-field-induced perturbation to the Larmor precession frequency. Our results for this frequency shift are consistent with zero; we find the corresponding ${}^{199}\mathrm{Hg}$ EDM $d_{\text{Hg}}=(-2.20\pm 2.75_{\mathrm{stat}}\pm 1.48_{\mathrm{syst}})\times {10}^{-30}e\mathrm{cm}$. We use this result to place a new upper limit on the ${}^{199}\mathrm{Hg}$ EDM $|d_{\text{Hg}}|<7.4\times {10}^{-30}e\mathrm{cm}$ (95% C.L.), improving our previous limit by a factor of 4. We also discuss the implications of this result for various $CP$-violating observables as they relate to theories of physics beyond the standard model.

Figure 1

Cross-sectional diagrams of the apparatus used to measure the EDM of ${}^{199}\mathrm{Hg}$ (not to scale). (a) Section of the vessel through the $y\text{−}z$ plane showing the HV cables, ground plane, and a cutaway view of the HV electrodes and feedthroughs. (b) Section through the $x\text{−}y$ plane showing the cylindrical three-layer magnetic shielding, the $\cos (\theta )$ magnet coil windings, and a diagram with 2 of the polarimeters used to observe signals from each of the 4 cells. The laser beams through the outer cells traverse the apparatus along the shield axis ($z$ axis), while the middle cell beams travel along the $x$ axis.

Figure 2

The signal obtained from a single photodiode for one pump-probe cycle. (a) A complete view of the signal. During optical pumping, the transmission through the cell increases, quickly saturating the detector. The laser power is reduced during the probe periods $A$ and $B$, which are analyzed to extract the phase difference accumulated between cells during the dark period. Individual Larmor oscillations are too rapid to be visible at this scale, but the exponential decay of the signal envelope can be seen. (b) An expanded view of the final 500 ms of the data train. The raw data points are connected by straight line segments to guide the eye; no fit is shown.

Figure 3

The measured EDM frequency shift ${\eta }_{\mathbf{B}}·\mathrm{\Delta }{\omega }_{\mathrm{EDM}}$ for each of the 252 runs in the final (reduced) data set. Each run is plotted on either the top or bottom chart. Top: the frequency shifts measured between $\pm 6\mathrm{kV}/\mathrm{cm}$. Bottom: shifts measured between $\pm 10\mathrm{kV}/\mathrm{cm}$.