Limits on Light-Speed Anisotropies from Compton Scattering of High-Energy Electrons

The possibility of anisotropies in the speed of light relative to the limiting speed of electrons is considered. The absence of sidereal variations in the energy of Compton-edge photons 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 limit of $1.6\times {10}^{-14}$ at 95% confidence level on a combination of the parity-violating photon and electron coefficients $({\tilde{\kappa }}_{o+}{)}^{YZ}$, $({\tilde{\kappa }}_{o+}{)}^{ZX}$, $c_{TX}$, and $c_{TY}$. This new constraint provides an improvement over previous bounds by 1 order of magnitude.

Figure 1

Schematic drawing of the tagging system.

Figure 2

Si $\mu$-strip count spectrum near the CE and the fitting function (see text) vs position $x$ and photon energy $\omega$. The three edges corresponding to the lines 364, 351, and 333 nm are clearly visible. The CE position $x_{\mathrm{CE}}$ is the location of the central line and is measured with a typical accuracy of $3\mu \mathrm{m}$.

Figure 3

Time evolution over a day of (a) ESRF beam intensity, (b) tagging-box temperature, (c) CE position and fitted curve [Eq. (5)], and (d) $\delta =x_{\mathrm{CE}}-x_{\mathrm{fit}}$. The error bars on position measurements are directly given by the CE fit.

Figure 4

Full set of data folded modulo a sidereal day (24 bins). The error bars are purely statistical and agree with the dispersion of the data points (${\chi }^2=1.04$ for the unbinned histogram). The shaded area corresponds to the region of nonexcluded signal amplitudes.