Limits on light-speed anisotropies from Compton scattering of high-energy electrons

The possibility of anisotropies in the speedof lightrelative to the limiting speed of electrons is considered. The absence of siderealvariations in the energy of Compton-edgephotons at the European Synchrotron Radiation Facility's GRAAL facility constrains such anisotropies representing the first nonthreshold collision-kinematics study of Lorentz violation. When interpreted within the minimal standard-model extension, this result yields the two- sided limitof 1.6x10{sup -14} at 95% confidence level on a combination of the parity-violating photon and electron coefficients ({ kappa}- tilde{sub o+}){sup YZ}, ({ kappa}- tilde{sub o+}){sup ZX}, c{sub TX}, and c{sub TY}. This new constraint provides an improvement over previous bounds by 1 order of magnitude.