What it is
ALPS II at DESY in Hamburg is a light-shining-through-a-wall experiment: as the preprint explains, light passing through a magnetic field would partly convert into axions if they exist, the axions would pass a wall that blocks the light, and in a magnetic field behind the wall some would convert back into photons. In its first science campaign, from February to May 2024, it searched for axions and similar lightweight particles and found no evidence for them. For pseudoscalar bosons like the axion, with masses below about 0.1 meV, it set a limit on the coupling strength to two photons of 1.5 × 10⁻⁹ GeV⁻¹ at a 95% confidence level, more than a factor of 20 better than all previous similar experiments. It also sets limits on photon interactions for scalar, vector and tensor bosons, and the campaign demonstrated stable operation and robust calibration of the complex experiment.
Why it matters
This is a laboratory-only test: unlike limits drawn from the Sun or the cosmos, its results do not depend on astrophysical or cosmological assumptions (the preprint). The optical system is now being upgraded, aiming for another two orders of magnitude in sensitivity, and the preprint states that ALPS II aims to surpass the limits from solar axion searches set by CAST.
Underlined numbers link to their source. Every metric and quoted figure is listed under Sources and data below.
Filed underDark Matter and Cosmic Phenomena, Particle physics theoretical and experimental studies, Neutrino Physics Research
