Physicists at the ALPS II collaboration reported the first results from their experiment searching for axions. Axel Lindner said, "Axions and similar very lightweight, very weakly interacting particles are prime candidates for physics beyond the Standard Model." The team found no evidence yet, but the work guides future dark matter searches.
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.
The search for symmetry breaking
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."
Generating and detecting
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.
In contrast, "the ALPS II approach is based on generating and detecting axions in the same experiments, so it does not require any such assumptions as the dark matter experiments," Lindner explains. "Hence, the upper limits from ALPS II are very robust results."
Light shining through a wall
The ALPS II experiment involves passing a powerful laser beam through a strong magnetic field, created 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, via the weak coupling between them.
Afterward, the laser encounters an opaque wall, which blocks its photons entirely. In contrast, the newly generated axions are able to pass straight through. On the other side of the wall, they encounter another 12 dipole magnets, which convert a small portion of them back into light with the exact same wavelength as the original laser.
This conversion takes place inside an optical cavity, which traps any regenerated photons between a pair of facing mirrors, allowing the light to build up. As this light leaks out of the cavity, it would interfere with a second laser, producing a faint yet detectable "beat signal" with a precisely known frequency.
Tightening the search
While the "light-shining-through-a-wall" approach was first described in 1987, ALPS II offers an unparalleled level of sensitivity: incorporating a total of 24 superconducting magnets and vastly boosting the regenerated signal with its optical cavity. A second cavity in front of the wall, now being installed, will boost axion production in a similar way.
Altogether, these improvements should raise the signal-to-noise ratio some 12 orders of magnitude above earlier experiments, giving a sensitivity comparable to astronomical observations.
In its latest study, the ALPS II collaboration has now reported results from the experiment's first science campaign, which ran from February to May 2024. "In its first science campaign, ALPS II has not found evidence for the existence of axions or similar hypothetical particles," Lindner says. "The 'excess' seen in the data was very likely traced back to stray light reaching the light detector, with an intensity of two photons per hour."
Even so, the experiment was more than 20 times more sensitive to axions than any similar experiment before it. With upgrades now underway, the collaboration hopes to improve this by another two orders of magnitude: perhaps bringing us a step closer to detecting the single most widely explored candidate for the universe's hidden mass.