Measurement of αs in e + e − Annihilation at Ecm = 29 GeV 1 TPC / Two - Gamma Collaboration

A measurement of the strong coupling constant αs using the event-shape variable y3 (the differential two-jet rate) in the reaction e+e− → hadrons is presented. The analysis is based on data from the TPC/Two-Gamma detector at the PEP e+e− storage ring taken between 1984 and 1986 at a center-of-mass energy of Ecm = 29 GeV. A value of αs(29 GeV) = 0.160 ± 0.012 is obtained, where the error is the quadratic sum of experimental and theoretical uncertainties. The procedure for determining αs is the same as that used by the ALEPH and TOPAZ experiments, which allows for a consistent comparison of the αs values obtained at different center-of-mass energies. The observed energy dependence “running” of αs is found to be in good agreement with the QCD prediction, and is clearly incompatible with a constant value.