Measurement of the H 3 Δ1 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)0028-083610.1038/s41586-018-0599-8] 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 H3Δ1 state, with an observation time τ≈1 ms for each molecule. We report here a new measurement of the lifetime of the ThO (H3Δ1) state, τH=4.2±0.5 ms. Using an apparatus within which τ≈τH will enable a substantial reduction in uncertainty of an eEDM measurement.