The ALPS (Any Light Particle Search) experiment, which stretches a total length of 250 meters, is looking for a particularly light type of new elementary particle. Using twenty-four 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. 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 these mysterious elementary particles and back into light again. “The idea for an experiment like ALPS has been around for over 30 years. By using components and the infrastructure of the former HERA accelerator, together with state-of-the-art technologies, we are now able to realize ALPS II in an international collaboration for the first time,” says Beate Heinemann, Director of Particle Physics at DESY. Helmut Dosch, Chairman of DESY’s Board of Directors, adds: “DESY has set itself the task of decoding matter in all its different forms. So ALPS II fits our research strategy perfectly, and perhaps it will push open the door to dark matter.”
The ALPS team sends a high-intensity laser beam along a device called an optical resonator in a vacuum tube, approximately 120 meters 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.”
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 meters 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-kilometer-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 minus 269 degrees Celsius 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. Ringwald says, “Experimental and theoretical physicists worked together very closely for ALPS. The result is an experiment which has a unique potential to discover axions, which we might eventually even use to search for high-frequency gravitational waves.”
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.”
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 Hanover, 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.
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.