Searching for traces of dark matter with neutron spin clocks

Cosmological observations of the orbits of stars and galaxies allow clear conclusions to be drawn about the attractive gravitational forces that act between celestial bodies.

The astonishing discovery: the visible matter is far from being sufficient to be able to explain the development or the movements of the galaxies. This suggests that there is another type of matter, hitherto unknown. As a result, in 1933 Swiss physicist and astronomer Fritz Zwicky deduced the existence of what is now called dark matter. Dark matter is a postulated form of matter that is not directly visible but interacts via gravity, and consists of about five times more mass than matter with which we are familiar.

Recently, following a precision experiment developed at the Albert Einstein Center for Fundamental Physics (AEC) at the University of Bern, an international research team managed to drastically narrow the scope of dark matter’s existence. With over 100 members, the AEC is one of the leading international research organizations in the field of particle physics. The findings of the team, led by Bern, have now been published in Physical examination letters.

The mystery surrounding dark matter

“What dark matter is actually made of is still unclear,” says Ivo Schulthess, who holds a doctorate. AEC student and lead author of the study. What is certain, however, is that it is not made of the same particles that make up the stars, planet Earth, or us humans. Worldwide, increasingly sensitive experiments and methods are being used to search for possible dark matter particles – so far, however, without success.

Certain hypothetical elementary particles, known as axions, are a promising class of possible candidates for dark matter particles. An important advantage of these extremely light particles is that they could simultaneously explain other important phenomena in particle physics that have not yet been understood.

Bern’s experience illuminates the darkness

“Thanks to many years of expertise, our team succeeded in designing and building an extremely sensitive measuring device, the Beam EDM experiment”, explains Florian Piegsa, professor of low energy and precision physics at the AEC. , who was awarded one of the prestigious ERC Starting Grants from the European Research Council in 2016 for his neutron research. If the elusive axions really exist, they should leave a characteristic signature in the measuring device.

“Our experiment allows us to determine the spin frequency of neutron spins, which move through a superposition of electric and magnetic fields,” says Schulthess. The spin of each individual neutron acts like a kind of compass needle, which rotates due to a magnetic field in the same way as the second hand of a wristwatch, but nearly 400,000 times faster.

“We precisely measured this rotational frequency and examined it for the smallest periodic fluctuations that would be caused by interactions with axions,” says Piegsa. The results of the experiment were clear: “The neutron spin frequency remained unchanged, which means that there is no evidence of axions in our measurement,” says Piegsa.

Parameter space was successfully reduced

The measurements, which were carried out with French researchers from the European Research Neutron Source at the Laue-Langevin Institute, allowed the experimental exclusion of a hitherto totally unexplored axion parameter space. It also proved possible to search for hypothetical axions that would be more than 1,000 times heavier than previously possible with other experiments.

“Although the existence of these particles remains mysterious, we have succeeded in excluding an important parametric space of dark matter”, concludes Schulthess. Future experiments can now build on this work. “Finally, answering the question of dark matter would give us significant insight into the foundations of nature and bring us closer to a full understanding of the universe,” says Piegsa.