The Nikhef eEDM experiment in Groningen, which is searching for asymmetry in the electron, has published its first EDM measurement and is now ready for rapid improvements.
The status of the experiment was presented in a recent article in the European Physical Journal D, along with an overview of the upcoming steps for significant future improvements. “The principle of the experiment, in which we test the mirror symmetry of a spinning electron, is simple; but the trick is to build the perfect mirror,” says Lorenz Willmann on behalf of the NL-eEDM team.
The Nikhef eEDM program has been working on the setup since 2018, which is being constructed in the laboratories in Groningen. Nikhef program leader Steven Hoekstra emphasizes that the Groningen experiment is entering a new phase.
“We are building our research program step by step. The published measurements mark the end of the construction phase. Now we will continue to refine the working setup.” With these steps, they expect to achieve a sensitivity comparable to other eEDM experiments worldwide.
The eEDM experiments revolve around measuring very small energy differences of an electron in parallel and anti-parallel electric and magnetic fields. The experiment in Groningen, with RUG, UvA and VU researchers involved, takes a unique approach. In the setup, electrons in barium fluoride (BaF) molecules are studied by using lasers to excite transitions between energy levels.
To this end, beams of molecules are brought into the most suitable quantum state for the measurements using the setup developed specifically for this purpose. The meter-long setup was fully assembled for the first time in 2024 to perform the initial measurements of the electron dipole moment in BaF.
Data were collected over a period of more than a day and a half, just before the entire experimental setup was moved to the new Feringa Building in Groningen. The measurements provide an upper limit for the eEDM: at most 2(3) × 10-25 ecm.
A non-zero electric dipole moment could shed new light on the differences between matter and antimatter. Dipole experiments are thus a valuable complement to accelerator experiments, such as those conducted at CERN.
An important reason for publishing this article is primarily the chosen approach, says Willmann. “We have developed a method that allows us to demonstrate that we can determine those electric and magnetic fields in the same measurement. This gives us confidence that, with the upcoming improvements, we will soon be able to measure much tighter limits. In terms of sensitivity, we will certainly come close to the other experiments.”
A triumph from an experimental standpoint, because it shows that all components of the complex experiment—and the theoretical understanding of it—are understood at least up to the current sensitivity. But modesty is called for: this first limit is currently about ten thousand times less stringent than the results of the most sensitive measurement to date, performed by a team using HfF ions in the US.
With this first limit using BaF, a new molecule is added to the existing measurements, which is important for distinguishing the underlying symmetry-breaking processes from one another.
The Nikhef physicists are searching for a signal from the electron dipole moment based on electron precession, the circular motion of the spin rotation axis of electrons. This motion is comparable to that of the tip of a spinning top that is not standing completely upright in the gravitational field. In the eEDM experiment, the electric field takes on the role of gravity.
The competing experiments do not have such a sensitive method for measuring that electric field and are more prone to systematic errors, says Willmann. “With our very precise approach, any deviation in the field is automatically eliminated,” he says.
Following the recently published measurements, the experimental setup was moved last year and reassembled in the new lab. This included a number of improvements to the molecular source and molecular beams. These are the result of developments over the past few years, the main focus of various PhD researchers.
The measurement method and technical innovations in the eEDM project will yield many sensitive measurements of the electron dipole moment over the course of next year, says program leader Hoekstra. “It is fascinating to see how these precise measurements of quantum systems are broadening research in particle physics. Innovations in both measurement techniques and the underlying theory are coming thick and fast.”
Willmann leaves open whether this will ultimately provide proof of a measurable electron dipole moment. “The experiment is, with all our capabilities, a question posed to nature. Whether or not there is a dipole moment, both are answers.”
