Progress in the ACME III Search for the Electron EDM

Electrostatic focusing of cold and heavy molecules for the ACME electron EDM search

“…This enhanced ground state population is then transferred into the quantum state used for lensing via STIRAP. The molecular beam is focused with an electrostatic hexapole lens, yielding a factor of 16 enhancement in signal size detected from the Q state, compared to when the lens is turned off, in a test beamline similar to the ACME III geometry 8 . Under these conditions, the transverse velocity and spatial distributions of the lens-focused beam are found to be closely comparable to those used in ACME II.…”

“…This improvement consists of two parts: a factor of 3.5 from the rotational cooling plus lens system if it was optimized for the shorter beamline in ACME II; and a factor of 1.3 from additional beam collimation by the lens in the expanded spin-precession region of ACME III. When combined with other demonstrated upgrades, including the longer coherence time to take advantage of the recently-measured H state lifetime [8], detectors with higher efficiency [35], and a reduction of technical noise [36], we project an improved statistical sensitivity to the electron EDM by potentially 30 times compared to the current best limit [3] from ACME II.…”

“…The H 3 Δ 1 state has similar Stark shifts to the Q state. However, its much shorter lifetime of ∼4 ms [8] would lead to significant loss from decay over the focusing beam path. In addition, dramatically higher laser power is needed to populate the H state via STIRAP [26], for the full distribution of molecules that can be captured by the lens.…”

“…One solution to increasing the number of useful molecules is to focus the ThO beam with an electromagnetic lens, which can exert a linear restoring force transverse to the beam and direct the originally diverging molecular trajectories into the downstream detection volume. Moreover, the upcoming version of ACME (referred to as ACME III) plans to operate with a longer spin-precession region, designed for an increased coherence time, based on the recently-measured longer lifetime of the H state [8]. This upgrade of a longer beamline makes collimation from a molecular lens even more important, since the detection volume is further downstream.…”

“…The inhomogeneity in photon collection efficiency results from the fact that the fluorescence collection optics are mounted only above the molecular beam. The functional form of the collection efficiency versus y position is from ray optics simulations, using the known geometry of the collection optics 8. It is useful to compare this with other configurations, using our validated trajectory simulations.…”