Over the past decade Eric Cornell and Jun Ye, both JILA Fellows and Professors at the University of Colorado Boulder, pioneered a new platform for measuring electron roundness: probing electrons in molecular-ions that are trapped by laboratory electric fields. Since trapped ions are fixed in space, they can be measured for much longer times than can molecules in molecular-beam experiments, where the molecules speed through the measurement apparatus. The downside of using ions is that electrostatic repulsion limits how tightly one can pack ions, limiting the density of molecules to a number small compared to that in molecular-beam experiments. Overall, trapped-ion and molecular-beam approaches have demonstrated comparable measurement sensitivity while no other approach has -as of 2020- achieved this level of sensitivity.

In a five-year, generation-III iteration of their experiment, Cornell and Ye will develop two primary improvements to their trapped-molecular-ion apparatus. First, they’ll switch from hafnium fluoride (HfF+) to thorium fluoride (ThF+) ions to take advantage of the better sensitivity of the Th-based molecules to the fluctuating particles that spoil electron roundness. Secondly, the PIs propose a new ‘bucket brigade’ apparatus that will measure many ion ‘buckets’ at once rather than in sequence. These changes are expected to improve the precision of electron roundness measurements by at least a factor of 10.

This project will produce high-profile publications, talks and posters at major conferences, and training for Ph.D. students and postdoctoral fellows.

A group at Imperial College London, led by Professors Michael Tarbutt and Edward Hinds, will evaluate the feasibility of using optically-trapped diatomic molecules to measure electron roundness. Optical traps are produced by intersecting laser beams to form an electromagnetic 'lattice' that holds molecules at fixed points in space. This allows the molecules to be measured for a long time. Further, optical traps can hold neutral molecules which are largely immune to electrostatic repulsion and so can be packed much more densely than can the molecular-ions used in competing experiments.

One challenge is that molecules must be very cold - typically μK temperatures- to be held in an optical trap. The primary goal of this three-year project is to demonstrate that a molecule well-suited for electron-roundness measurements can be cooled to the ultra-low temperatures required for optical trapping. Tarbutt and Hinds will use cryogenic-molecular-beam techniques (laser ablation, then cooling via collisions with a cryogenic buffer gas) to produce a beam of ytterbium fluoride molecules that are cold enough to allow laser-based slowing and cooling techniques to take over and cool the molecules further to the μK temperatures required for trapping. A favorable outcome for this project would position the team for a follow-on project that improves eEDM sensitivity by a factor of up to 1000.

This project will produce a number of papers on laser slowing and cooling of an electric dipole moment relevant molecule, as well as training for postdocs and graduate students.

Professors Nicholas Hutzler at Caltech and John Doyle at Harvard head the PolyEDM collaboration, one which proposes two innovations for electric dipole moment measurement: optical trapping and the use of polyatomic molecules. Optical traps use intersecting laser beams to form an electromagnetic 'lattice' that can hold neutral molecules at fixed points in space. This promises high molecular density and long measurement times, both important to improved measurement precision. Using a polyatomic molecule is a second important innovation in electron-roundness measurements.

The leading EDM measurement groups worldwide use molecules with specialized properties that allow one to distinguish the tiny, sought-after signal from the many sources of experimental noise and error that could mask a new-particle signal. These specialized properties (omega-doublet-like states) have until recently not been identified in molecules that can be laser cooled, leaving scientists with a choice: either use molecules with these beneficial specialized properties or molecules with a different set of benefits associated with optical cooling and trapping. Importantly, the PolyEDM collaboration realized that simple polyatomic molecules (like ytterbium hydroxide) can provide the best of both worlds; this molecule can be laser cooled/trapped and it offers the specialized molecular properties necessary to efficiently combat noise and systematic error.

The primary goals for this three-year project are to demonstrate that ytterbium hydroxide can be laser-cooled to the very low (μK) temperatures required for laser cooling, and to efficiently put these molecules into the quantum state useful for EDM measurements. This project could improve eEDM sensitivity by a factor of one hundred in 6 years.

The project will produce high profile publications, talks and posters at major conferences, and training for postdocs and graduate students.

The EDM³ collaboration is led by Professors Eric Hessels at York University, Amar Vutha at the University of Toronto, and Jaideep Singh at Michigan State University. This team proposes to use polar molecules trapped in cryogenic solid argon for a precise electron electric dipole moment measurement. This approach could improve measurement sensitivity by providing a way to hold many molecules at fixed points in space so they can be measured for a long time. In this sense the proposed approach offers similar benefits to optical trapping, yet it’s even more promising since this approach aims to measure a few billion molecules in a matrix, compared to a few million envisioned by optical-trapping experiments, and a few thousand in electrostatic-trap experiments. Additionally, the matrix provides a way to capture flipped orientations of a molecule and this amounts to a powerful tool for eliminating noise and systematic error.

While a promising approach, it's possible that vibrations of the solid or other effects will make a precision measurement in the system impractical. This 3-year project will directly address this uncertainty. Hessels, Vutha, and Singh will create a beam of barium fluoride (BaF) molecules, implant the molecules into a solid argon (Ar) matrix while it is being grown, and then perform spectroscopic measurements on the embedded molecules to see if a precision EDM measurement is practical. In-situ diagnostics will be used to probe the growth and implantation process and a range of different growth and annealing schedules will be used to optimize the platform. The PIs estimate that they could improve eEDM sensitivity by a factor of 200 in 6 years.

The project will produce several research papers, talks and posters at relevant conferences, and training for students and postdocs.

Professor Matthew Reece, a theoretical physicist at Harvard University, will lead an effort to improve physicists’ understanding of how electron electric dipole moment experiments (EDMs) can help guide particle physics. An electron could have a tiny EDM for one of two reasons. First, any undiscovered particles that contribute to the EDM could be very heavy. Increasingly heavy particles make ever smaller contributions to an EDM, so an experimentally determined upper limit on the EDM could mean that particles below a certain energy/mass do not exist. The second possible reason for a tiny EDM is that the undiscovered particles participate only weakly in the types of interactions to which EDMs are sensitive.

This theory project will improve our understanding of the types of particles that participate only weakly in EDM-sensitive interactions. In practice this means building models of new physics and using them to compute the EDM of fundamental particles like an electron. Ultimately this will help physicists interpret experimental upper limits on the electron EDM, to understand whether an upper-limit is more likely a statement about how heavy undiscovered particles might be or a statement about the types of interactions possible for these particles. Further, the particles and interactions at the heart of this study are also of interest to other branches of particle physics such as collider physics and neutrino physics, and this work could spark a new dialog between disparate branches of particle physics.

Professor Reece will work with a postdoc and a graduate student and the project will produce at least three papers as well as presentations and posters at relevant major conferences.