A Measurement of the QCD Colour Factors using Event Shape Distributions at sqrt(s)=14 GeV to 189 GeV
Abstract
Measurements of the QCD colour factors CA and CF and of the number of active quark flavours nf in the process e+e- -> hadrons at high energy are presented. They are based on fits of O(alphaS**2)+NLLA QCD calculations to distributions of the event shape observables 1-T, C, BT and BW measured at centre-of-mass energies from 14 GeV to 189 GeV. Hadronisation effects are approximated with power correction calculations which also depend on the QCD gauge structure. In this approach potential biases from hadronisation models are reduced. Our results for individually measured quantities are nf = 5.64 +- 1.35, CA = 2.88 +- 0.27 and CF = 1.45 +- 0.27 in good agreement with QCD based on the SU(3) symmetry group where nf=5 for the energies considered here, CA=3 and CF=4/3. From simultaneous fits of CA and CF we find CA = 2.84 +- 0.24 and CF = 1.29 +- 0.18, which is also in good agreement with the QCD expectation.
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