New bounds on heavy axions with an X-ray free electron laser

We present new exclusion bounds obtained at the European X-ray Free Electron Laser facility (EuXFEL) on axion-like particles (ALPs) in the mass range 10⁻³\;eV mₐ 10⁴\;eV, which is relatively unconstrained by laboratory searches. Our experiment exploits the Primakoff effect via which photons can, in the presence of a strong external electric field, decay into axions, which then convert back into photons after passing through an opaque wall. While similar searches have been performed previously at a 3ʳᵈ generation synchrotron {Yamaji:2018ufo} our work demonstrates improved sensitivity, exploiting the higher brightness of X-rays at EuXFEL.