An experiment of X-ray photon-photon elastic scattering with a Laue-case   beam collider
  We report a search for photon-photon elastic scattering in vacuum in the X-ray region at an energy in the center of mass system of \omega_{cms} =6.5 keV for which the QED cross section is \sigma_{QED} =2.5 \times 10^{-47} m^2. An X-ray beam provided by the SACLA X-ray Free Electron Laser is split and the two beamlets are made to collide at right angle, with a total integrated luminosity of (1.24 \pm 0.08) \times 10^{28} m^{-2}. No signal X rays from the elastic scattering that satisfy the correlation between energy and scattering angle were detected. We obtain a 95% C.L. upper limit for the scattering cross section of 1.9 \times 10^{-27} m^2 at \omega_{cms}=6.5 keV. The upper limit is the lowest upper limit obtained so far by keV experiments. 
T. Yamaji, T. Inada, T. Yamazaki, T. Namba, S. Asai, T. Kobayashi, K.   Tamasaku, Y. Tanaka, Y. Inubushi, K. Sawada, M. Yaba...
physics.ins-det hep-ex