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

In the process e(+)e(-) --> hadrons, one of the effects of gluon emission is to modify the (1 + cos(2)theta) 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 sigma(L)/sigma(tot) = 0.0127 +/- 0.0016 +/- 0.0013 at the parton level, in good agreement with the expectation of QCD computed to O(alpha(s)(2)). Comparisons 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. (C) 1998 Elsevier Science B.V. All rights reserved.