World's Most Sensitive Lab Search for Axions Finds Nothing and Narrows the Hunt
Germany's ALPS II experiment shot a laser through a wall of magnets to look for the hypothetical particle, and its first results set robust new limits.
Physicists running the ALPS II experiment in Germany have reported first results from what they call the most sensitive instrument of its kind in the hunt for axions, a hypothetical particle that could explain dark matter. They found no evidence for axions in this first campaign, but the result sharply narrows where the particle can hide.
The collaboration is headed by Axel Lindner at the DESY research center. The work has been posted as a preprint on arXiv and will be published in Physical Review Letters.
The axion was proposed to solve a puzzle in the Standard Model. The weak nuclear force is known to violate charge-parity, or CP, symmetry, meaning the laws of physics are not identical if a particle is swapped with its antiparticle and mirrored in space. Strangely, the strong nuclear force, which binds quarks into protons and neutrons, seems to respect CP symmetry perfectly. In 1977 Roberto Peccei and Helen Quinn proposed a mechanism to explain why, and Steven Weinberg and Frank Wilczek showed it required a new, extremely light particle that interacts only faintly with ordinary matter. That particle was named the axion.
Physicists later realized axions could also make up the dark matter that holds galaxies together. "Axions and similar very lightweight, very weakly interacting particles are prime candidates for physics beyond the Standard Model of particle physics," Lindner said. "They could explain the dark matter in our universe, perhaps also dark energy."
No experiment has yet found one. Many searches look for axions in the dark matter believed to surround the Milky Way, while some of the tightest limits come from studies of stars, including the sun. Those approaches generally lean on assumptions about conditions in the early universe that cannot be tested directly.
ALPS II avoids that by making and detecting axions in the same apparatus. A powerful laser beam passes through a strong magnetic field produced by a string of 12 superconducting dipole magnets. If axions exist, the field would convert a small fraction of the laser's photons into axions. The beam then hits an opaque wall that stops every photon, but axions would pass straight through. On the far side, a second magnet section can turn some of them back into light that a sensitive detector could register. A signal appearing from behind a solid wall would be a discovery.
"The ALPS II approach is based on generating and detecting axions in the same experiment, so it does not require any such assumptions as the dark matter experiments," Lindner said. "Hence, the upper limits from ALPS II are very robust results."
A null result is still progress. Each run rules out a larger range of how strongly axions could couple to light, and the collaboration says the first results will guide future runs. The search for a particle that could answer two of physics' biggest questions at once continues.
