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

TL;DR

This paper demonstrates electrostatic focusing and improved cooling of ThO molecules, significantly enhancing molecular flux and population, which could lead to over tenfold improvements in the sensitivity of the ACME electron EDM experiment.

Contribution

It introduces electrostatic focusing of ThO molecules and an upgraded cooling scheme, enabling more efficient use of molecules for electron EDM measurements.

Findings

Factor of 16 enhancement in molecular flux with electrostatic focusing

3.5 times increase in ground state population due to improved cooling

Projected over tenfold improvement in statistical sensitivity for future ACME experiments

Abstract

The current best upper limit for electron electric dipole moment (EDM), $|d_e|<1.1\times10^{-29}\,$e$\cdot$cm ($90$% confidence), was set by the ACME collaboration in 2018. The ACME experiment uses a spin-precession measurement in a cold beam of ThO molecules to detect $d_e$. An improvement in statistical uncertainty would be possible with more efficient use of molecules from the cryogenic buffer gas beam source. Here, we demonstrate electrostatic focusing of the ThO beam with a hexapole lens. This results in a factor of $16$ enhancement in the molecular flux detectable downstream, in a beamline similar to that built for the next generation of ACME. We also demonstrate an upgraded rotational cooling scheme that increases the ground state population by $3.5$ times compared to no cooling, consistent with expectations and a factor of $1.4$ larger than previously in ACME. When combined with other demonstrated improvements, we project over an order of magnitude improvement in statistical sensitivity for the next generation ACME electron EDM search.