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 acting between celestial bodies.

The surprising finding: Visible matter is far from being able to explain the growth or motions of galaxies. This suggests that there is another, so far unknown, type of matter. Hence, in the year 1933, the Swiss physicist and astronomer Fritz Zwicky deduced the existence of what is now known as dark matter. Dark matter is a hypothesized form of matter that is not directly visible but interacts through gravity and is about five times more massive than the matter we are familiar with.

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 significantly narrow the field for the existence of dark matter. With more than 100 members, 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 the Physical Review Letters.

The mystery surrounding dark matter

“What dark matter actually consists of is still completely unclear,” explains Ivo Schulthess, Ph.D. student at AEC and the 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. Around the world, 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 potential candidates for dark matter particles. A major advantage of these ultralight particles is that they could simultaneously explain other important phenomena in particle physics that are not yet understood.

The Bern experiment sheds light on the darkness

“Thanks to many years of expertise, our team was able to design and build an extremely sensitive measuring device — the Beam EDM experiment,” explains Florian Piegsa, Professor of Low Energy and Precision Physics at 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 values ​​actually exist, they should leave behind a distinctive signature on the measuring device.

“Our experiment enables us to determine the spin frequency of neutron spins, which move through a superposition of electric and magnetic fields,” explains Schulthess. The spin of each individual neutron acts as a kind of compass needle, which rotates due to a magnetic field similar to the second hand of a wristwatch – but almost 400,000 times faster.

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

The parameter space has been narrowed successfully

The measurements, which were carried out with researchers from France at the European Research Neutron Source at the Laue-Langevin Institute, allowed the experimental exclusion of a previously completely unexplored parameter space of the axes. It also proved possible to search for hypothetical axes 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 successfully ruled out a significant parameter space of dark matter,” concludes Schulthess. Future experiments can now build on this work. “Finally, answering the question of dark matter would give us important insight into the fundamentals of nature and take us a big step closer to fully understanding the universe,” explains Piegsa.