Small-Scale Fundamental Physics

ACME Collaboration

A close up of the optics of the ACME II experimental apparatus. The ACME collaboration attempts to detect new particles through measuring the distribution of electrical charge in the electrons in a cold beam of thorium oxide molecules.

goal statement

To discover new fundamental physics

The Electric Dipole Moment Opportunity

The dominant method for discovering new fundamental particles in physics has been the construction of extremely large, expensive facilities, such as particle colliders.  These massive instruments crash tiny particles together at breakneck speeds, momentarily creating the super high-energy conditions theory tells us will form this or that fundamental particle.  Physicists then observe the collision in the hope the theorized particle shows up. The problem with such facilities is that they are extremely expensive—perhaps prohibitively so—and take over a decade to construct. Advancement in particle physics has correspondingly slowed in the supercollider era. There’s a need for new avenues of discovery that can test theory more rapidly and at lower cost.

Quantum theory has opened just such an avenue.  We do not, quantum theory tells us, have to create Big-Bang-like conditions to detect the presence of new fundamental particles.  All fundamental particles fluctuate into and out of existence, though for very brief periods.  What we need is an instrument sensitive enough to detect them.  That instrument is the electron.

Electrons are charged particles that, in isolation, are perfectly round. When a particle pops into existence near an electron, however, it deforms the electron’s charge distribution, warping it in ways systematically related to the mass of the fluctuating particle.  If we can detect how deformed the electron’s charge is, we can find out something about the particle that deformed it. We know how deformed the electron should be due to fluctuations from the types of fundamental particles currently known to physics. As yet undiscovered particles can make electrons even more deformed, so measuring electron roundness—inside physics this is called measuring the electron’s electric dipole moment (EDM)—can reveal signatures of new particles.

Grants in Small-Scale Fundamental Physics, made in partnership with the Gordon and Betty Moore Foundation and the National Science Foundation, support a range of different efforts to find signs of new particles by measuring electron roundness. Five teams will each pursue a different experimental approach to measuring electron roundness and one theory team will improve our understanding of the connection between electron roundness and certain classes of hypothesized new particles. Success is far from assured.  Constructing an effective detector requires using complicated techniques to exercise precise control over the electrons in ultracold molecules or solids, so that their charge distribution can be accurately measured.  The potential rewards, however, are high.  If successful, the experiments could reveal signatures of entirely new building blocks of the universe, and at a fraction of the cost associated with a super-collider, perhaps pointing the way towards a new era of discovery in particle physics.

Supported Projects


ACME III (the Advanced Cold Molecule Electron electric dipole moment search) is a Yale-Harvard-Northwestern collaboration focused on detecting signatures of new fundamental particles by measuring the roundness of the electron. Led by Professors David DeMille at Yale, John Doyle at Harvard, and Gerald Gabrielse at Northwestern, the team developed cold-molecular-beam techniques and used them to take electric dipole moment measurements to a new level of precision. A cold-molecular-beam is a dense stream of (here thorium oxide) molecules made super-cold through collisions with the cold molecules of a cryogenic buffer gas. The high molecular density and cold temperatures obtainable with this technique led to world-leading electron electric dipole moment measurement precision in the first and second iterations of the ACME experiment, and phase III looks to build upon this record of success. Over a five-year period, ACME III will increase the number of molecules measured and the duration of each measurement, boosting the overall sensitivity of the experiment, compared to ACME II, by a factor of 30.

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

University of Colorado, Boulder

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.

Imperial College London

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.

California Institute of Technology

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.

York University

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.

Harvard University

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.


The Foundation is not currently accepting grant applications in Small-Scale Fundamental Physics. Additional grants may be made in future years pending evaluation of the outcomes of the initial round of supported projects.

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