Measurement of the Longitudinal Cross-Section using the Direction of the Thrust Axis in Hadronic Events at LEP

TL;DR

This paper measures the longitudinal cross-section in e+e- hadronic events at LEP, testing QCD predictions by analyzing the thrust axis orientation and comparing results with theoretical calculations and Monte Carlo models.

Contribution

It provides the first precise measurement of the longitudinal cross-section ratio at LEP, validating QCD predictions to order alpha_s^2.

Findings

Measured ratio of longitudinal to total cross-section: 0.0127±0.0016±0.0013

Good agreement with QCD calculations at order alpha_s^2

Studied the dependence of the longitudinal cross-section on thrust, offering a new QCD test.

Abstract

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.