This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Physicists at the ALPS II collaboration have reported the first results from the Any-Light-Particle-Search (ALPS) II experiment: the most sensitive instrument of its kind in the search for hypothetical "axions." Headed by Axel Lindner at the DESY Research Centre in Germany, the collaboration found no evidence for axions in this initial campaign, but its results offer valuable guidance for future runs: tightening the search for one of the most promising candidates for dark matter. The research has been posted as a preprint on arXiv and will be published in Physical Review Letters.
If a particle were simultaneously swapped with its corresponding antiparticle and flipped spatially into its mirror image, it might be intuitive to predict that the laws of physics governing its behavior would remain exactly the same. But in the weak nuclear force, which drives some types of radioactive decay, this principle of "charge-parity (CP)-symmetry" can be violated. CP-violation is now built into the Standard Model, which presents our most up-to-date understanding of the universe's most fundamental forces and particles.
Strangely, however, CP-symmetry seems to hold perfectly in the strong nuclear force, which binds quarks together to form protons, neutrons and other, more short-lived hadrons. In 1977, Roberto Peccei and Helen Quinn presented a new theory which aimed to explain why CP-symmetry isn't violated by the strong force. Steven Weinberg and Frank Wilczek then showed that their idea required an entirely new particle, which doesn't appear in the Standard Model.
Linked to symmetry breaking at extremely high energies, this particle would naturally cancel out any CP-violation in the strong force. Named the "axion," this hypothetical particle would be extremely light and would interact only very weakly with ordinary matter and light, making it incredibly difficult to detect. Building on this theory, physicists proposed that axions could also account for one of the deepest mysteries in cosmology.
"Axions and similar very lightweight, very weakly interacting particles are prime candidates for physics beyond the Standard Model of particle physics," Lindner says. "They could explain the dark matter in our universe, perhaps also dark energy." Like any theory in physics, the axion's existence can't be confirmed until it is detected directly. In the decades since Peccei and Quinn's prediction, physicists have sought these elusive particles using a wide array of large-scale experiments.
Many have searched for axions within the dark matter thought to surround our galaxy, while some of the tightest limits have come from observations of stars, including the sun. So far, none of these studies have provided any concrete evidence for axions. On top of this, they have generally relied on assumptions about the conditions of the early universe, which can't be tested directly.
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