ALPS II reports no axion signal from its first science campaign

Their theory required a particle that does not appear in the Standard Model, as Steven Weinberg and Frank Wilczek subsequently showed. Linked to a change in symmetry at extremely high energies, this particle would naturally cancel CP violation in the strong force.

Physicists named it the “axion.” The particle would be extremely light and interact only very weakly with ordinary matter and light, a combination that makes it extraordinarily difficult to detect.

“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.”

Those possibilities speak directly to one of cosmology’s deepest mysteries.

How have scientists searched for them?

Decades of large-scale experiments have sought axions in the dark matter thought to surround our galaxy. Other searches have used observations of stars, including the Sun, to establish some of the tightest limits. No study has yet produced concrete evidence.

Those approaches generally depend on assumptions about early-universe conditions that scientists cannot test directly. ALPS II instead looks for axions by attempting to create and detect them within the same experimental setup, so its upper limits do not require those cosmological assumptions. Lindner characterized the resulting upper limits as very robust.

How does light pass through a wall?

ALPS II sends a powerful laser beam through a strong magnetic field produced by a string of 12 superconducting dipole magnets. If axions exist, that field converts a small fraction of the laser’s photons into axions through their weak interaction.

An opaque wall then blocks the original photons completely, while the newly produced axions pass through it. On the far side, another 12 dipole magnets convert a small portion of those axions back into light with exactly the same wavelength as the original laser.

The regenerated photons build inside an optical cavity, an arrangement that traps light between a pair of facing mirrors. As this light leaks out, it interferes with a second laser and creates a faint beat signal at a precisely known frequency.

The light-shining-through-a-wall method was first described in 1987. ALPS II uses 24 superconducting magnets in total and adds an optical cavity that greatly boosts the regenerated signal.

What will the upgrades change?

The collaboration is now installing a second optical cavity in front of the wall. It should boost axion production in a similar way that the receiving cavity boosts regenerated light.

The collaboration expects these improvements to raise the signal-to-noise ratio—how clearly a signal stands out from background noise—by some 12 orders of magnitude over earlier experiments. That is roughly one trillion times, giving ALPS II sensitivity comparable to astronomical observations.

The first campaign was already more than 20 times as sensitive as any similar experiment before it. The collaboration hopes to improve that performance by another two orders of magnitude, or roughly 100-fold, during upcoming runs.

Only a direct signal could confirm the particle’s existence. Even so, the upgrades may bring the search closer to detecting what remains the most widely explored candidate for the universe’s hidden mass.

The forthcoming Physical Review Letters paper is listed as D C Brotherton et al., “Any Light Particle Searches with ALPS II: first science results” (2026), DOI 10.1103/61sw-k3bc. The arXiv preprint carries DOI 10.48550/arxiv.2512.14110.