Measurement of the longitudinal cross-section using the direction of the thrust axis in hadronic events at LEP

In the process e+e- to hadrons, one of the effects of gluon emission is to modify the 1+cos(theta)**2 form of the angular distribution of the thrust axis, an effect which may be quantified by the longitudinal cross-section. Using the OPAL detector at LEP, we have determined the longitudinal to total cross-section ratio to be 0.0127+-0.0016+-0.0013 at the parton level, in good agreement with the expectation of QCD computed to Order(alpha_s**2) Comparisions at the hadron level with Monte Carlo models are presented. The dependence of the longitudinal cross-section on the value of thrust has also been studied, and provides a new test of QCD.

Note:

16 pages, LaTeX, 4 eps figures and 3 colour eps figs included, submitted to Phys. Lett. B Report-no: CERN-EP/98-124

electron positron: colliding beams
electron positron: annihilation
Z0: electroproduction
Z0: hadronic decay
hadron: multiple production
gluon: emission
cross section: longitudinal
total cross section
thrust
angular distribution

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