Abstract

We present the first determination of the strong coupling constant from the three-jet rate in e+e- annihilation at LEP, based on a next-to-next-to-leading-order (NNLO) perturbative QCD prediction. More precisely, we extract αs(MZ) by fitting perturbative QCD predictions at O(αs3) to data from the ALEPH experiment at LEP. Over a large range of the jet-resolution parameter ycut, this observable is characterized by small nonperturbative corrections and an excellent stability under renormalization scale variation. We find αs(MZ)=0.1175±0.0020(expt)±0.0015(theor), which is more accurate than the values of αs(MZ) from e+e- event-shape data currently used in the world average.