Measurement of the $H{}^3{\mathrm{\Delta }}_1$ radiative lifetime in ThO

The best limit on the electron electric dipole moment (eEDM) comes from the ACME II experiment [Nature (London) 562, 355 (2018)] which probes physics beyond the standard model at energy scales well above 1 TeV. ACME II measured the eEDM by monitoring electron spin precession in a cold beam of thorium monoxide (ThO) molecules in the metastable $H{}^3{\mathrm{\Delta }}_1$ state, with an observation time $\tau \approx 1$ ms for each molecule. We report here a new measurement of the lifetime of the ThO ($H{}^3{\mathrm{\Delta }}_1$) state, ${\tau }_H=4.2\pm 0.5$ ms. Using an apparatus within which $\tau \approx {\tau }_H$ will enable a substantial reduction in uncertainty of an eEDM measurement.

Figure 1

Experimental setup (not to scale) used to probe the $H$-state lifetime.

Figure 2

(a) Optical pumping excitation of the $H$ state in an electric field. (b) Detection of fluorescence after a second excitation.

Figure 3

Semilog plot of the intensity ratios as a function of transit time between the excitation regions and fluorescence detector, with a fit to Eq. (2). The colors indicate two data sets taken at widely separated times, before (orange) and after (blue) an ablation target change that resulted in a change in ThO velocity. Uncertainty in the transit times arising from uncertainty in the positions of the excitation regions are addressed in Sec. 3.

Figure 4

Average absorption near the molecule source (blue, left) and fluorescence in the detection region (orange, right) traces show the transit time and the longitudinal velocity dispersion of the molecular pulses. The peak time (dotted) and average time (solid) differ slightly. The displayed traces are averaged from the entire second data set. The height of the absorption signal has been inverted and rescaled so it can be compared to the detected fluorescence signal.

Figure 5

Projected EDM sensitivity gains over ACME II given the measured lifetime ${\tau }_H$ for a perfectly collimated molecular beam (red upper curve). The bands represent the effect of the uncertainty in ${\tau }_H$, and the dashed lines indicate the coherence times for ACME II and the projected ACME III. For a diverging molecular beam in an apparatus that is made longer without increasing its radial dimensions, the longer ${\tau }_H$ increases the sensitivity by a factor of 2 (orange lower curve). The sensitivity improves by up to 2.6 due to the effective collimation provided by the addition of an electrostatic lens for the molecules (blue middle curve). (The additional sensitivity gain of 3.5 because the lens also captures more molecules is not included.)