Physicists involved in the Any-Light-Particle-Search (ALPS) II experiment have released their first results from the search for axions, hypothetical particles that could help explain dark matter. While the team were unable to find any evidence of axions, the results demonstrate the experiment’s potential and what it will look for in the future. Led by Axel Lindner at the DESY Research Centre in Germany, ALPS II uses the most sensitive equipment of its type.Axion search backgroundThis search is related to the charge parity (CP) symmetry. CP symmetry can be broken by the weak nuclear force, which is responsible for certain types of radioactive decay. However, the strong nuclear force that binds together quarks in protons, neutrons and other hadrons appears to maintain CP symmetry.In 1977 Roberto Peccei and Helen Quinn put forward a theory to explain why there appeared to be no CP violations in the strong nuclear force. Steven Weinberg and Frank Wilczek later showed that this theory predicted the existence of a new particle, called the axion. This particle would have very low mass and have only very weak interactions with normal matter and light. They could also help explain dark matter – unseen matter thought to make up much of the universe. Lindner says they are also the leading candidates for physics beyond the Standard Model and could maybe explain dark energy.Past searches for axions have looked at dark matter in galaxies and stars like the Sun, however there is no clear evidence. ALPS II tries to produce and detect axions in one experiment.Axion detectionTo detect axions ALPS II uses a technique called light shining through a wall. The laser beam is shone through a magnetic field created by 12 superconducting dipole magnets. If axions exist, some of the photons from the laser will turn into axions due to their weak interaction with the magnetic field.The photons are blocked by an opaque wall, but the axions can pass through. These axions then enter another set of 12 dipole magnets that may convert some axions back to photons with the original laser wavelength. These photons then enter an optical cavity with mirrors facing each other creating a build-up of the signal. Any light escaping will cause interference with a second laser allowing us to see a weak beat signal with a known frequency. Finding this signal will indicate that there are axions.The optical cavity helps amplify the signal of the reconverted photons. Another cavity will be added in front of the wall to help create more axions. With these improvements the signal to noise ratio should increase by around 12 orders of magnitude compared to previous experiments. This puts its sensitivity close to astronomical observations.During the first science run between February and May 2024 the team did not find any axions or similar hypothetical particles. There was a slight increase in the data which was most likely due to stray light hitting the detector. This light had an intensity of 2 photons per hour.However ALPS II was over 20 times more sensitive than any previous similar experiments. With these upgrades the team hope to increase the sensitivity by another 2 orders of magnitude to help test one of the leading theories to explain the hidden mass in the universe and give more accurate measurements in the future.The research is currently available as a preprint on arXiv and is expected to be published in Physical Review Letters.