Measurement of the Electromagnetic Coupling at Large Momentum Transfer

We report the first purely electroweak measurement of the strengthening of the electromagnetic coupling, ${\alpha }_{\mathrm{QED}}$ with increasing momentum transfer $Q^2$, by comparing the process $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ with the process $e^{+}{e}^{-}\to e^{+}{e}^{-}{\mu }^{+}{\mu }^{-}$. The data were accumulated at an average center-of-mass energy $〈\sqrt{s}〉=57.77\mathrm{}\mathrm{GeV}$ using the TOPAZ detector at the TRISTAN $e^{+}{e}^{-}$ collider. We measure ${\alpha }_{\mathrm{QED}}^{-1\mathrm{}}(Q^2$) to change from its known value of ${\alpha }_{\mathrm{QED}}^{-1\mathrm{}}\cong 137.0$ at $Q^2=0\mathrm{}\mathrm{to}128.5\pm 1.8\left(\mathrm{stat}\right)\pm 0.7\left(\mathrm{syst}\right)$ at $Q^2=(57.77\mathrm{GeV}/c{)}^2$. This result agrees with electroweak predictions.