No X-Rays shining through a wall: A search for heavy, axion-like particles with a Free Electron Laser

The axion was posed as a solution to the CP problem of QCD, but axion-like particles (ALPs) also arise in string theory and are a dark matter (DM) candidate. Most laboratory axion searches have concentrated on the 0.001-0.1 meV mass range however there is growing interest in heavier (DFSZ) axions above 10 meV which avoid the cosmological domain wall catastrophe [1].

We describe a new search for axions at EuXFEL Hamburg, sensitive to a mass range including 1 meV-1 eV, which is largely unconstrained by laboratory experiments. Our experiment exploits the Primakoff effect, via which (in the presence of strong external fields) photons can decay into axions and then reconvert back into photons. This has previously been employed in experiments with lasers and external magnetic fields; however using X-rays it is possible to increase the detection sensitivity by using the electric field present within a crystalline material. X-Ray searches were previously performed at a 3rd generation synchrotron [2]; however limited flux prevented probing down to DM relevant couplings. Our work is a first step in developing a platform with improved sensitivity thanks to an increase in brightness by ~10¹⁰ at EuXFEL. Initial work has confirmed previous bounds on the coupling strength and probed a previously unexplored mass range. In future we hope to probe down to a coupling relevant to QCD axions in the keV mass range.

[1] K. Beyer & S. Sarkar SciPost Phys. 15 (2023) 003

[2] T. Yamaji et al., Phys. Lett. B 782 (2018) 523