Any Light Particle Searches with ALPS II: first science results

TL;DR

ALPS II at DESY conducts a pure laboratory search for WISPs using a resonantly enhanced light-shining-through-a-wall approach. By employing long straightened magnet strings and high-finesse optical cavities, the experiment translates photon-WISP conversions into measurable regenerated photons behind a wall, enabling model-independent limits on axions, hidden photons, and tensor bosons. In the first science campaign (Feb–May 2024), no evidence of WISPs was found, but the axion-photon coupling sensitivity improved by over a factor of 20 relative to prior LSW experiments, with stringent limits across scalar, pseudoscalar, vector, and tensor sectors. The collaboration demonstrates stable operation and calibration, and plans major optical upgrades to push sensitivity toward astrophysical bounds and beyond existing laboratory constraints.

Abstract

The light-shining-through-a-wall experiment ALPS II at DESY in Hamburg searched for axions and similar lightweight particles in its first science campaign from February to May 2024. No evidence for the existence of such particles was found. For pseudoscalar bosons like the axion, with masses below about 0.1 meV, we achieved a limit for the di-photon coupling strength of 1.5e-9 1/GeV at a 95% confidence level. This is more than a factor of 20 improvement compared to all previous similar experiments. We also provide limits on photon interactions for scalar, vector and tensor bosons. An achievement of this first science campaign is the demonstration of stable operation and robust calibration of the complex experiment. Currently, the optical system of ALPS II is being upgraded aiming for another two orders of magnitude sensitivity increase.

Figures (6)

• Figure 1: The optical system implemented in the first science run of ALPS II (taken from aaron). "FSR" denotes the free spectral range of the regeneration cavity, "North Left", "Central Hall" and "North Right" refer to the three cleanrooms at the ALPS II magnet strings. The black bar on the "central optical bench" indicates the light-tight wall housing a shutter which is opened for system checks and calibration. See the text for a brief explanation and aaron for details. Twelve HERA dipole magnets each surround the production area and the regeneration cavity.
• Figure 2: Taken from aaron: normalized spectral power from measurements on $\mathrm{{PD_{science}}}$ determined for frequencies around the signal's heterodyne frequency for the polarization $\mathrm{}{\gamma_\parallel}$ (Table \ref{['tab:expresults']}). An LSW signal would only show up in the frequency bin centered at 0 Hz. The observed broader distribution is likely caused by stray-light.
• Figure 3: Limits on pseudoscalar bosons: "Previous laboratory" (grey) summarizes previous results from ALPS Ehret:2010mh and OSQAR OSQAR:2015qdv; CAST (light-blue) is taken from CAST:2024eil and references therein. The golden axion band shows an "artist's" view on the approximate range given by KSVZ- and DFSZ-inspired models PhysRevLett.43.103SHIFMAN1980493DINE1981199Zhitnitsky:1980tq, while the yellow range refers to a more recent model Sokolov:2021ydn. The green area shows the result of this analysis; the purple line indicates the ALPS II prospects.
• Figure 4: Limits on scalar bosons: "Fifth force" (tan) is taken from Adelberger_2003KONOPLIV2011401 with the scaling $\mathrm{}{Q_e \sim 1/500}$ used in Arvanitaki_2016, "Previous laboratory" (grey) summarizes results from ALPS Ehret:2010mh and OSQAR OSQAR:2015qdv; "CAST" (light-blue) is taken from CAST:2024eil and references therein. The green area shows the result of this analysis; the purple line indicates the ALPS II prospects.
• Figure 5: Limits on hidden photons: "Previous laboratory" (grey) summarizes results from CROWS Betz_2013 and ALPS Ehret:2010mh; "CAST" data (light-blue) are taken from Redondo_2008. The green area shows the result of this analysis; the purple line indicates the ALPS II prospects.