Publikationsserver der RWTH Aachen University
Suche nach dem Higgs-Boson im Vier-Jet-Kanal mit dem L3-Detektor bei LEP
Abstract
dc:descriptionThis thesis presents the search for the Higgs boson which is predicted by the Standard Model of Particle Physics in order to endow bosons and fermions with their observed masses. The search is performed using data taken by the L3 experiment at the Large Electron Positron Collider (LEP) during the years 1998 until 2000. An integrated luminosity of 628/pb was collected in a center-of-mass energy range from 189 GeV to 209 GeV. This thesis is devoted to the analysis of the reaction e+ e- --> HZ--> bb- qq-. The observed four-jet final state results from the Higgs boson decaying to a bb- pair and a hadronic Z decay. It is the most likely of various possible Higgs decay channels. After an initial selection requiring high multiplicity, four-jet-like events a neural network is used to separate signal from background. It exploits b-tagging information together with kinematic and event shape variables. Subsequently the neural network output is folded with the reconstructed Higgs mass from a kinematic fit in order to get a Higgs mass dependent final discriminant which is used to search for the Higgs boson for each mass hypothesis in the range between 85 GeV and 115 GeV. The number of observed candidates in the analyzed channel is in good agreement with the expectation for the Standard Model background. To determine a lower limit on the mass of the Higgs boson a likelihood ratio is used. This procedure optimizes the discovery potential of the analysis in the sense that it both maximizes the probability to discover an existent signal and the probability to exclude a false signal hypothesis. By applying this method a lower limit on the mass of the Higgs boson of m_H > 106.4 GeV has been set at 95 % confidence level. If a 100 GeV Higgs boson had existed it would have been observed with a significance of 3.4 sigma. Although the L3 four-jet analysis reveals no indication of a signal, the LEP combined results show a preference for a Higgs boson with a mass of 115.6 GeV. The main origin of this 2.1 sigma excess compared to the background expectation is the ALEPH four-jet channel. Because the LEP operation has been stopped at the end of the year 2000, it will be up to future colliders like Tevatron and LHC to decide whether LEP has just missed the Higgs boson discovery.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2002
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Wienemann, Peter
- Contributors dc:contributor
-
- Böhm, Albrecht
Subjects
dc:subject × 2Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger