Measurement of the Electromagnetic Coupling at Large Momentum Transfer

We report the first purely electroweak measurement of the strengthening of the electromagnetic coupling, α_QED with increasing momentum transfer Q², by comparing the process e⁺e^--->μ⁺μ^- with the process e⁺e^--->e⁺e^-μ⁺μ^-. The data were accumulated at an average center-of-mass energy <s> = 57.77 GeV using the TOPAZ detector at the TRISTAN e⁺e^- collider. We measure α^-1_QED\(Q²) to change from its known value of α^-1_QED≅137.0 at Q² = 0 to 128.5+/-1.8\(stat\)+/-0.7\(syst\) at Q² = \(57.77 GeV/c\)². This result agrees with electroweak predictions.