The ALPS ‘Light shines through a wall’ experiment began searching for dark matter


May 23, 2023

(Nanowerk NewsThe world’s most sensitive model independent experiment to search for special light particles, which may consist of dark matter, began today at DESY in the form of the ALPS II ‘light shining through walls’ experiment. Scientific calculations predict that this ominous form of matter will occur five times more frequently in the universe than ordinary visible matter. But until now, no one has been able to identify the particles of this substance; the ALPS trial can now provide such evidence. The ALPS experimental magnet array in the HERA tunnel ALPS experimental magnet array in HERA tunnel: In this section of the magnet, powerful laser beams are reflected back and forth, from which axions are supposed to form. (Image: DESY, Marta Mayer)

The ALPS (Search for Any Light Particle) experiment, which spans 250 meters, is looking for a new type of extremely light elementary particle. Using twenty-four recycled superconducting magnets from a HERA accelerator, intense laser beams, precision interferometry and highly sensitive detectors, an international team of researchers set out to search for these so-called axions or axion-like particles. Such particles are believed to react only very weakly with known types of matter, meaning they cannot be detected in experiments using accelerators.

Therefore, ALPS uses a completely different principle to detect them: in a strong magnetic field, photons – that is, light particles – can be transformed into these mysterious elementary particles and back into light again.

“The idea of ​​an experiment like ALPS has been around for more than 30 years. Using the components and infrastructure of previous HERA accelerators, together with state-of-the-art technology, we are now able to bring ALPS II to international collaboration for the first time,” said Beate Heinemann, Director of Particle Physics at DESY. Helmut Dosch, Chairman of DESY’s Board of Directors, added: “DESY has set the task of decoding material in all its different forms. So ALPS II fits perfectly into our research strategy, and maybe it will open the door to dark matter.”

The ALPS team sent a high-intensity laser beam along a device called an optical resonator in a vacuum tube, about 120 meters long, where it bounced back and forth and which was surrounded by twelve HERA magnets arranged in a straight line. . If a photon turns into an axion in a strong magnetic field, that axion can pass through the opaque wall at the end of the magnetic lines. Once it passes through the wall, it will enter another magnetic path almost identical to the first. Here, the axion can then turn back into a photon, which the detector will eventually capture. A second optical resonator was installed here to increase the probability of the axion turning back into a photon by a factor of 10,000. That is, if light arrives behind a wall, it must be an axion in between.

“However, despite all our technical tricks, the chances of a photon going around the axis and back again are minuscule,” says Axel Lindner of DESY, project leader and spokesperson for the ALPS collaboration, “like rolling 33 dice and they all come out the same.”


The ALPS principle is explained in this animation.

For the experiment to really work, the researchers had to tune all of the different equipment components for maximum performance. The light detector is so sensitive that it can detect one photon per day. The mirror system’s precision for light is also record-breaking: the distance between the mirrors must remain constant within a fraction of the atomic diameter relative to the laser’s wavelength. And the superconducting magnets, each nine meters long, generate a magnetic field of 5.3 Tesla in a vacuum tube, more than 100,000 times the strength of Earth’s magnetic field.

The magnets were retrieved from the HERA proton accelerator’s 6.3-kilometer ring and recycled for the ALPS project. The magnets were initially curved on the inside and had to be straightened for the experiment to accommodate more laser light; and the safety equipment for operating it in superconducting conditions at minus 269 degrees Celsius has been completely revised. The ALPS experiment was originally proposed by DESY theorist Andreas Ringwald, who also supports the theoretical motivation for the experiment with his calculations about the expansion of the Standard Model.

Ringwald said, “Experimental and theoretical physicists worked very closely together for ALPS. The result is an experiment with the unique potential to find an axis, which we might eventually use to search for high-frequency gravitational waves.”

The axion search will initially start in an attenuated operating mode, simplifying “backlight” searches that may incorrectly indicate the presence of an axion. The experiment will reach full sensitivity in the second half of 2023. The mirror system will be upgraded in 2024, and alternative light detectors may also be installed at a later time. Scientists hope to publish the first results from ALPS in 2024. Lindner believes, “Even if we don’t find light particles with ALPS, the experiment will shift the exclusion limit for ultra-light particles by a factor of 1000.” Panoramic shot of the 250 meter long ALPS experiment Panoramic shot of the 250 meter long ALPS experiment. In the center is the first magnet installed in the ALPS II tunnel. The people involved have signed it. (Image: DESY, Marta Mayer)

In all, about 30 scientists have joined the international ALPS collaboration. They hail from seven research institutes: in addition to DESY, the Max Planck Institute of Gravitational Physics (Albert Einstein Institute), the Institute of Gravitational Physics at Leibniz University in Hanover, Cardiff University (UK), University of Florida (Gainesville, Florida, USA), Johannes Gutenberg University in Mainz, the University of Hamburg and the University of Southern Denmark (Odense) are all involved.

The researchers are already making plans for the time after their current axion search. For example, they want to use ALPS to find out whether magnetic fields affect the propagation of light in a vacuum, as Euler and Heisenberg predicted decades ago. And the researchers also want to use the experimental setting to detect high-frequency gravitational waves.

What are axions?

The axis is a hypothetical elementary particle. They are part of the physical mechanism postulated by the theoretical physicist Roberto Peccei and his colleague Helen Quinn in 1977 to solve the problem of the strong interaction – one of the four fundamental forces of nature. In 1978, theoretical physicists Frank Wilczek and Steven Weinberg connected a new particle with this Peccei-Quinn mechanism. Because these particles would “clean” the theory, Wilczek named them “actions” after the 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 will solve a whole suite of problems currently puzzling physicists, including being a candidate for building dark matter. According to current calculations, this dark matter should be five times more abundant in the universe than normal matter.


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