Determination of αs from a measurement of the direct photon spectrum in Γ(1S) decays

Abstract

Using the ARGUS detector at the DORIS II e +e − storage ring we have measured direct photons from the decay ???(1 S)→ γgg. The ratio R γ = Γ(???(1S)→ γgg)/ Γ(???(1S)→ ggg)=(3.00±0.13±0.18)% has been determined, from which we deduce values of the strong coupling constant α s=0.225±0.011±0.019 and the QCD scale parameter Λ MS =115±17±28 MeV defined in the modified minimal-subtraction scheme. The shape of the measured spectrum clearly rules out the predictions of the lowest order QCD calculations.