Measurement of the Direct Photon Spectrum in Υ(1S) Decays

Original · EN

Using data taken with the CLEO II detector at the Cornell Electron Storage Ring, we have determined the ratio of branching fractions: Rγ ≡ Γ(Υ(1S) → γgg)/Γ(Υ(1S) → ggg) = (2.75 ± 0.04(stat.) ± 0.15(syst.))%. From this ratio, we have determined the QCD scale parameter ΛMS (defined in the modified minimal subtraction scheme) to be ΛMS= 233 ± 11 ± 59 MeV, from which we determine a value for the strong coupling constant αₛ(MΥ₍₁ₛ₎) = 0.163 ± 0.002 ± 0.014, or αₛ(MZ) = 0.110 ± 0.001 ± 0.007.