We report here the first experimental result for the anisotropy of the one-way maximum attainable speed of the electron, $vec{Delta c_{1,e}}$, obtained via the study of a sidereal time dependence of a difference between the electron and positron beam momenta in the CESR storage ring at Cornell University. At 95 percent confidence, an upper limit for the component of $Delta vec {c}_{1,e}/c$ perpendicular to Earths rotational axis is found to be $5.5 times 10^{-15}$.
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