Axions are believed to interact very weakly with known forms of matter, rendering them virtually undetectable through conventional accelerator experiments. Instead, ALPS utilizes a different principle: in a robust magnetic field, photons, or light particles, could potentially morph into axions and subsequently revert back to light.

The ALPS experiment, long envisioned by physicists, has now been brought to life with the assistance of former HERA accelerator components and state-of-the-art technologies, creating an innovative international collaboration.

Helmut Dosch, Chairman of DESY's Board of Directors, described the ALPS II project as a perfect fit for DESY's research strategy, which seeks to decode all forms of matter. He expressed optimism that the project might provide insights into the mystery of dark matter.

The experimental setup of ALPS involves a high-intensity laser beam sent along an optical resonator within a 120-metre long vacuum tube. The beam is reflected back and forth within the tube, flanked by twelve linearly arranged HERA magnets. If a photon were to transform into an axion in the magnetic field, it could pass through an opaque wall at the end of the magnet line, enter another magnetic track, then revert to a photon, detectable at the end.

Despite advanced technical implementations, the likelihood of a photon turning into an axion and back is minimal, akin to rolling 33 dice and having all land on the same face, according to DESY's Axel Lindner, project leader of the ALPS collaboration.

The research team had to optimize the apparatus components for the experiment to function. The detector, capable of sensing a single photon per day, is extremely sensitive. The mirror system, maintaining a precise and constant distance relative to the laser's wavelength, set a record for light precision. Moreover, the nine-meter-long superconducting magnets generate a magnetic field over 100,000 times stronger than the Earth's in the vacuum tube.

Initial tests will commence with a simplified search for "background light" that could mistakenly signify axions' presence. Full sensitivity is expected in the latter half of 2023, and upgrades to the mirror system and potential installation of an alternative light detector are planned for 2024. Regardless of the outcome, ALPS will push the boundaries of ultra-light particle detection.

The ALPS collaboration is an international endeavor, comprising approximately 30 scientists from seven research institutions worldwide. After the current search for axions, the team plans to investigate whether a magnetic field impacts light propagation in a vacuum, a theory postulated decades ago by Euler and Heisenberg, and to detect high-frequency gravitational waves using the experimental setup.

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.

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