Measurement of the Direct Photon Spectrum in ${\rm Y}(1S)$ Decays

Abstract

Using data taken with the CLEO II detector at the Cornell Electron Storage Ring, we have determined the ratio of branching fractions: $R_{\gamma }\equiv \Gamma ({\rm Y}(1S)\to \gamma gg)/\Gamma ({\rm Y}(1S)\to ggg)=(2.75\pm 0.04(stat.)\pm 0.15(syst.))$. From this ratio, we have determined the QCD scale parameter ${\Lambda }_{\overline{MS}}$ (defined in the modified minimal subtraction scheme) to be ${\Lambda }_{\overline{MS}}=233\pm 11\pm 59$ MeV, from which we determine a value for the strong coupling constant ${\alpha }_s(M_{{\rm Y}(1S)})=0.163\pm 0.002\pm 0.014$, or ${\alpha }_s(M_Z)=0.110\pm 0.001\pm 0.007$.

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