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 axionlike particles in the mass range 10^{-3}  eV≲m_{a}≲10^{4}  eV. 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 third-generation synchrotron [Yamaji et al., Phys. Lett. B 782, 523 (2018)PYLBAJ0370-269310.1016/j.physletb.2018.05.068], our work demonstrates improved sensitivity, exploiting the higher brightness of x-rays at EuXFEL.