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DESY News: ‘Light shining through a wall’ experiment ALPS starts searching for dark matter
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News from the DESY research centre
‘Light shining through a wall’ experiment ALPS starts searching for dark matter
The world’s most sensitive model-independent experiment to search for particularly light particles, of which dark matter might be composed, starts today at DESY in the form of the ‘light shining through a wall’ experiment ALPS II. Scientific calculations predict that this ominous form of matter should occur five times as often in the universe as normal, visible matter. Until now, however, no one has been able to identify particles of this substance; the ALPS experiment could now furnish such evidence.

Magnet row of the ALPS experiment in the HERA tunnel: In this part of the magnets, intense laser light is reflected back and forth, from which axions are supposed to form. Photo: DESY, Marta Mayer
The ALPS team sends a high-intensity laser beam along a device called an optical resonator in a vacuum tube, approximately 120 metres in length, in which the beam is reflected backwards and forwards and which is enclosed by twelve HERA magnets arranged in a straight line. If a photon were to turn into an axion in the strong magnetic field, that axion could pass through the opaque wall at the end of the line of magnets. Once through the wall, it would enter another magnetic track almost identical to the first. Here, the axion could then change back into a photon, which would be captured by the detector at the end. A second optical resonator is set up here to increase the probability of an axion turning back into a photon by a factor of 10 000. This means, if light does arrive behind the wall, it must have been an axion in between. “However, despite all our technical tricks, the probability of a photon turning into an axion and back again is very small,” says DESY’s Axel Lindner, project leader and spokesperson of the ALPS collaboration, “like throwing 33 dice and them all coming up the same.”

The search for axions will initially begin in an attenuated operating mode, simplifying the search for “background light” that might falsely indicate the presence of axions. The experiment is due to achieve full sensitivity in the second half of 2023. The mirror system is to be upgraded in 2024, and an alternative light detector can also be installed at a later time. The scientists expect to publish the first results from ALPS in 2024. Lindner is convinced, “Even if we don’t find any light particles with ALPS, the experiment will shift the exclusion limits for ultra-light particles by a factor of 1000.”

Panoramic photo of the 250-metres long ALPS experiment. In the centre is the first magnet installed in the tunnel for ALPS II. The people involved have signed it. Photo: DESY, Marta Mayer
The researchers are already making plans for the time after their current search for axions. For example, they want to use ALPS to find out whether a magnetic field influences the propagation of light in a vacuum, as predicted decades ago by Euler and Heisenberg. And the researchers also want to use the experimental setup to detect high-frequency gravitational waves.
What are axions?
Axions are hypothetical elementary particles. They are part of a physical mechanism postulated by the theoretical physicist Roberto Peccei and his colleague Helen Quinn in 1977 in order to solve a problem of the strong interaction – one of the four fundamental forces of nature. In 1978, the theoretical physicists Frank Wilczek and Steven Weinberg linked a new particle to this Peccei-Quinn mechanism. Since this particle would “clean up” the theory, Wilczek named it “axion” after a detergent. A number of different extensions of the Standard Model of particle physics predict the existence of axions or axion-like particles. If they do exist, they would solve a whole series of problems currently puzzling physicists, including being candidates for the building blocks of dark matter. According to current calculations, this dark matter should be around five times as abundant in the universe as normal matter.
More information about ALPS:
Youtube video: https://www.youtube.com/watch?v=qwbGDRTQG48
Website about Dark Matter and ALPS: https://darkmatter-alps.desy.de/dark-matter.html
Animation: https://www.youtube.com/watch?v=Eyq9G8Znc7U
Introduction to ALPS: https://www.youtube.com/watch?v=BDnBpbl09FA
Introduction to ALPS (vertical): https://www.youtube.com/shorts/guuxUZSlz1Y
ALPS website: https://www.desy.de/research/facilities__projects/alps_ii/index_eng.html