Search for Photon-Photon Elastic Scattering in the X-ray Region
  We report the first results of a search for real photon-photon scattering using X rays. A novel system is developed to split and collide X-ray pulses by applying interferometric techniques. A total of $6.5\times10^{5}$ pulses (each containing about $10^{11}$ photons) from an X-ray Free-Electron Laser are injected into the system. No scattered events are observed, and an upper limit of $1.7\times 10^{-24}$ ${\rm m^{2}}$ (95% C.L.) is obtained on the photon-photon elastic scattering cross section at 6.5 keV. 
T. Inada, T. Yamaji, S. Adachi, T. Namba, S. Asai, T. Kobayashi, K.   Tamasaku, Y. Tanaka, Y. Inubushi, K. Sawada, M. Yabash...
hep-ex physics.ins-det physics.optics