A limit on the anisotropy of the one-way maximum attainable speed of the electron

We report here the first experimental result for the anisotropy of the one-way maximum attainable speed of the electron, $\overrightarrow{\mathrm{\Delta }{c}_{1,e}}$, 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 $\mathrm{\Delta }{\vec{c}}_{1,e}\mathrm{/}c$ perpendicular to Earth’s rotational axis is found to be $5.5\times {10}^{-15}$.