Measurement of {alpha}{sub s} in e{sup +}e{sup {minus}} annihilation at E{sub cm} = 29 GeV

A measurement of the strong coupling constant {alpha}{sub s} using the event-shape variable y{sub 3} (the differential two-jet rate) in the reaction e{sup +}e{sup {minus}} {yields} hadrons is presented. The analysis is based on data from the TPC/Two-Gamma detector at the PEP e{sup +}e{sup {minus}} storage ring taken between 1984 and 1986 at a center-of-mass energy of E{sub cm} = 29 GeV. A value of {alpha}{sub s}(29 GeV) = 0.160 {+-} 0.012 is obtained, where the error is the quadratic sum of experimental and theoretical uncertainties. The procedure for determining {alpha}{sub s} is the same as that used by the ALEPH and TOPAZ experiments, which allows for a consistent comparison of the {alpha}{sub s} values obtained at different center-of-mass energies. The observed energy dependence {open_quotes}running{close_quotes} of {alpha}{sub s} is found to be in good agreement with the QCD prediction, and is clearly incompatible with a constant value.