‘Light shining through a wall’ experiment ALPS starts searching for dark matter

World’s most sensitive instrument of its kind is to produce axions

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. Scientists believe that this mysterious form of matter is five times more abundant in the universe than normal, visible matter. Until now, however, no one has been able to observe particles of this substance; the ALPS experiment could now furnish such evidence. Key contributions to the novel experiment come from researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) and the Institute for Gravitational Physics at Leibniz Universität Hannover.

The ALPS (Any Light Particle Search) experiment, which stretches a total length of 250 metres, is looking for a particularly light type of new elementary particle. Using 24 recycled superconducting magnets from the HERA accelerator, an intense laser beam, precision interferometry and highly sensitive detectors, the international research team wants to search for these so-called axions or axion-like particles. Such particles are believed to react only extremely weakly with known kinds of matter, which means they cannot be detected in experiments using accelerators.

Gravitational-wave technology for the search for dark matter

ALPS is therefore resorting to an entirely different principle to detect them: in a strong magnetic field, photons – i.e. particles of light – could be transformed into mysterious axions and back into light again. Only now, more than 30 years after the initial idea, can an international collaboration now realize the experiment.

The researchers from the Hannover institutes provide key parts of the experimental setup and performed pioneering laboratory experiments. “Like the detection of gravitational waves, the ALPS-II measurement must be absolutely reliable and accurate from start to finish. At AEI Hannover and Leibniz Universität, we have decades of experience in building and operating the required high-precision, high-power laser light sources,” says Benno Willke, leader of the Lasers and Squeezed Light research group at the Max Planck Institute for Gravitational Physics. “At ALPS II, our wealth of experience benefits the search for dark matter. We developed and provided the high-power laser system and associated optics for the experiment, and set it up and commisioned it at DESY.”

Light through the wall

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. The Hannover physicists have developed special control electronics to ensure that the laser is always perfectly tuned to the resonator and that the probability of axions being created from the laser photons is as high as possible.

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. “Incidentally, the first ideas for this second resonator were discussed in Hannover back in the early 1990s,” explains Guido Müller, director of the “Precision Interferometry and Fundamental Interactions” department at AEI Hannover and professor of physics at the University of Florida. “We revisited these ideas 15 years ago and subsequently developed the optical design and detection method for ALPS.”

DESY’s Axel Lindner, project leader and spokesperson of the ALPS collaboration explains that the probability that a photon changes into an axion and back again is nevertheless very small – like throwing 33 dice and them all coming up the same.

From the start of the measurements to first results

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.”

The researchers are already making plans for the time after their current search for axions. They also want to use ALPS to detect high-frequency gravitational waves. Additionally, they want to use the experimental setup to find out whether a magnetic field influences the propagation of light in a vacuum. This magnetic vacuum birefringence in was theoretically predicted decades ago. Together with colleagues from the University of Florida, the Hannover researchers are currently developing an experimental setup to be installed and commissioned at DESY in the future to study magnetic birefringence in vacuum at ALPS.

The ALPS collaboration

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

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.

Technical background information

In order for the experiment to actually work, the researchers had to tweak all the different components of the apparatus to maximum performance. The light detector is so sensitive that it can detect a single photon per day. The precision of the system of mirrors for the light is also record-breaking: the distance between the mirrors must remain constant to within a fraction of an atomic diameter relative to the wavelength of the laser. And the superconducting magnets, each nine metres long, generate a magnetic field of 5.3 Tesla in the vacuum tube, more than 100,000 times the strength of the Earth’s magnetic field. The magnets were taken from the 6.3-kilometre-long proton ring of the HERA accelerator and upcycled for the ALPS project. The magnets were originally curved on the inside and had to be straightened for the experiment so that they could store more laser light; and the safety equipment for operating them under superconducting conditions at –269 °C has been completely revised. The ALPS experiment was originally proposed by DESY theoretician Andreas Ringwald, who also underpinned the theoretical motivation for the experiment with his calculations on extending the Standard Model.