A study of QCD structure constants and a measurement of {Mathematical expression} at LEP using event shape observables

Abstract

The dependence of event shape cross sections on the QCD structure constants CA, CF and TF is studied using data from the OPAL detector at LEP. The observables Thrust, Heavy Jet Mass, Total and Wide Jet Broadening are used. They allow the use of O(αs 2), resummed NLLA, and combined O(αs 2) plus resummed NLLA QCD calculations so that a comparison between the different approaches can be performed. The measured values of the structure constants are found to be consistent with standard QCD based on SU(3) and five active quark flavours. A measurement of the strong coupling constant using NLLA QCD calculations alone results in {Mathematical expression}, which complements our previous determinations..