Searching for traces of dark matter with neutron spin clocks


An international research team has succeeded in significantly narrowing the scope for the existence of dark matter, with the use of a precision experiment developed at the University of Bern, and carried out at the Institut Laue-Langevin (ILL).

The results have just been published in PRL. They make an important contribution to the search for dark matter particles, of which little remains known, and significantly narrow the scope for the existence of dark matter.

Cosmological observations of the orbits of stars and galaxies enable clear conclusions to be drawn about the attractive gravitational forces that act between celestial bodies. The astonishing finding: visible matter is far from sufficient to explain the development or movements of galaxies. This suggests that there exists another, so far unknown, type of matter. Accordingly, in 1933, the Swiss physicist and astronomer Fritz Zwicky inferred the existence of what is known now as dark matter - a postulated form of matter that isn’t directly visible but interacts via gravity,and is thought to contribute approximately 85% of the total mass in the universe.

The mystery surrounding dark matter

“What dark matter is actually made of is still completely unclear,” explains Ivo Schulthess, a PhD student at the Albert Einstein Center for Fundamental Physics (AEC) in Bern (Switzerland) and the lead author of the study. What is certain, however, is that it is not made from 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 – until now, however, without success.

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

Bern experiment sheds light on the darkness

Thanks to many years of expertise, the team has succeeded in designing and building an extremely sensitive measurement apparatus – the Beam EDM experiment. If the elusive axions actually exist, they should leave behind a characteristic signature in the measurement apparatus.

“Our experiment enables us to determine the rotational 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 like 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 which would be caused by interactions with axions,” explains Florian Piegsa (AEC) who has initiated this research. The results of the experiment were clear: “The rotational frequency of the neutrons remained unchanged, which means that there is no evidence of axions in our measurement,” says Piegsa.

Parameter space successfully narrowed down

The measurements were carried out with researchers from France at the Institut Laue-Langevin, the European neutron source.

"This experiment profits from the world’s highest cold neutron flux available at the cold neutron beam facility PF1B, as well as the flexibility of this facility”, explains Torsten Soldner, ILL responsible for PF1B, who took part in the experiment.

The measurements excluded a region of the parameter space of axions that had previously been unexplored by laboratory experiments. It was possible to search for hypothetical axions which would be more than 1,000 times heavier than tested in other experiments.

“Although the existence of these particles remains mysterious, we have successfully excluded an important 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 a significant insight into the fundamentals of nature and take us a big step closer to a complete understanding of the universe,” explains Piegsa.

ILL Contacts: Dr. Torsten Soldner, Dr. Estelle Chanel


Schulthess I. et al., New limit on axion-like dark matter using cold neutrons, Physical Review Letters 129, 191801 (2022). DOI: 10.1103/PhysRevLett.129.191801

ILL Instrument: Polarised cold neutron beam facility PF1B

Note: The Albert Einstein Center for Fundamental Physics (AEC) in Bern (Switzerland)

With more than 100 members, the AEC is one of the leading international research organisations in the field of particle physics. It focuses on experimental and theoretical particle physics and its applications (such as medical physics), as well as associated spin-off and outreach activities.

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