Measurement of the Electromagnetic Coupling at Large Momentum Transfer

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