Neutron spin clocks are a proposed method for detecting dark matter. The idea is that neutrons have a small magnetic moment, or “spin,” that can be used to measure the strength of a magnetic field. If dark matter particles interact with normal matter through a magnetic interaction, then a neutron spin clock could potentially detect their presence. However, this is currently a theoretical idea and has not been proven to work in practice. More research is needed to determine if neutron spin clocks can be used to detect dark matter.
An international research team has significantly narrowed the scope for the existence of dark matter using a precision experiment developed at the University of Bern. The experiment was carried out at the European Research Neutron Source at the Institute Laue-Langevin in France, and it contributes significantly to the search for these particles, about which little is known.
Cosmological observations of star and galaxy orbits allow for precise conclusions about the attractive gravitational forces that act between celestial bodies. The astonishing discovery: visible matter is insufficient to explain the formation or movement of galaxies. This indicates the existence of a previously unknown type of matter. As a result, the Swiss physicist and astronomer Fritz Zwicky postulated the existence of what is now known as dark matter in 1933. Dark matter is a hypothetical form of matter that is not directly visible but interacts with us via gravity and has approximately five times the mass of matter that we are familiar with.
Our experiment allows us to determine the rotational frequency of neutron spins as they move through a superposition of electric and magnetic fields. We precisely measured this rotational frequency and examined it for the smallest periodic fluctuations that would be caused by interactions with the axions.
Ivo Schulthess
Recently, following a precision experiment developed at the Albert Einstein Center for Fundamental Physics (AEC) at the University of Bern, an international research team succeeded in significantly narrowing the scope for the existence of dark matter. With more than 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 the journal Physical Review Letters.
The mystery surrounding dark matter
“What dark matter is actually made of is still completely unknown,” says Ivo Schulthess, the study’s lead author and a Ph.D. student at the AEC. What is certain 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, without success.
Certain hypothetical elementary particles known as axions are a promising category of potential candidates for dark matter particles. One significant advantage of these extremely light particles is that they may explain other important phenomena in particle physics that are currently unknown.
Bern experiment sheds light on the darkness
“Thanks to many years of expertise, our team has succeeded in designing and building an extremely sensitive measurement apparatus — the Beam EDM experiment,” explains Florian Piegsa, Professor for 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 research with neutrons. If the elusive axions actually exist, they should leave behind a characteristic signature in the measurement apparatus.
“Our experiment allows us to determine the rotational frequency of neutron spins as they move through a superposition of electric and magnetic fields,” Schulthess explains. The spin of each individual neutron acts as a kind of compass needle, rotating due to a magnetic field in the same way that the second hand of a wristwatch does – 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 the axions,” Piegsa explains. The experiment produced clear results: “The rotational frequency of the neutrons remained unchanged, indicating that there is no evidence of axions in our measurement,” Piegsa says.
Parameter space successfully narrowed down
The measurements, made in collaboration with French researchers at the European Research Neutron Source at the Institute Laue-Langevin, enabled the experimental exclusion of a previously unexplored parameter space of axions. It was also demonstrated that it was possible to search for hypothetical axions that were more than 1,000 times heavier than was previously possible with other experiments.
“While the existence of these particles remains a mystery, we have successfully excluded an important parameter space of dark matter,” Schulthess concludes. This work can now be built upon in future experiments. “Finally answering the question of dark matter would provide us with significant insight into the fundamentals of nature and bring us one step closer to a complete understanding of the universe,” Piegsa explains.
