Probing keV mass QCD axions with the SACLA X-ray free electron laser

TL;DR

This paper reports an experimental search for QCD axions and axion-like particles using the SACLA X-ray free electron laser, significantly extending bounds on axion-photon coupling across a broad mass range, including the QCD axion region.

Contribution

It introduces a novel light-shining-through-wall experiment utilizing Laue crystals and the Bormann effect to probe axions from keV to tens of eV masses, improving existing bounds.

Findings

Extended bounds on axion-photon coupling by over an order of magnitude.

Achieved sensitivity reaching the QCD axion coupling prediction in the 3460-3480 eV mass range.

Provided the most stringent laboratory constraints for certain axion mass ranges.

Abstract

Axions are hypothetical particles, proposed to account for the invariance of CP symmetry in quantum chromodynamics. While axions and axion-like-particles are well-motivated by string theory and beyond-Standard-Model extensions, they have remained elusive to experimental searches even after significant effort over many decades. Building on a recent development using an X-ray free electron laser to search for cosmologically favoured axions of mass $m_{a} \lesssim 0.01$ eV, we extend previous bounds on the ALP-photon coupling, $g_{a\gamma\gamma}$, by over an order of magnitude. We exploit the Bormann effect of Laue crystals in a light-shining-through-wall experiment, with broad sensitivity to $m_a \lesssim$ 22 eV. Moreover for $m_{a} \in$ (3460, 3480) eV our sensitivity reaches down to the QCD axion coupling prediction, providing the most stringent laboratory constraints in this mass range.