Results of a Search for Paraphotons with Intense X-ray Beams at SPring-8

A search for paraphotons, or hidden U(1) gauge bosons, is performed using an intense X-ray beamline at SPring--8. "Light Shining through a Wall" technique is used in this search. No excess of events above background is observed. A stringent constraint is obtained on the photon--paraphoton mixing angle, $χ< 8.06\times 10^{-5}\ (95%\ {\rm C.L.})$ for $0.04\ {\rm eV}<m_{γ^{\prime}} < 26\ {\rm keV}$.