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UBC Theses and Dissertations

A measurement of the strong coupling constant using two jet event rates at 91 Gev Jones, Matthew


An analysis of hadronic Z° decays observed in the OPAL detector at LEP is presented. The analysis involves the selection of hadronic Z° decays from the set of OPAL data and the selection of well resolved charged tracks and electromagnetic energy clusters observed in each event. These are then used to determine the relative rates of events with 2-, 3-, 4- and 5-jets using a number of different jet definition schemes. These rates are then compared with theoretical predictions, computed perturbatively to second order in the coupling constant as using the theory of Quantum Chromodynamics. By fitting the theoretical predictions to the data, the QCD parameters Λ and µ are determined and a value of the strong coupling constant is obtained for each jet definition scheme. In the analysis, the experimental uncertainties in the measured n-jet event rate distributions are estimated. A particular emphasis is placed on the description of the techniques used to correct the measured distributions for the limited acceptance and response of the detector and for the unknown process of hadronization. The applicability of the corrections for detector acceptance and response are justified and an examination of the uncertainties in the measured values of αs due to the unknown level of virtuality of the primary partons is presented. It is found that the measured values of αs are consisted with other measurements made at LEP energies and that the values determined in each jet definition scheme are consistent with each other.

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