Joana Ferreira Montenegro.
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Nationaal instituut voor subatomaire fysica National institute for subatomic physics |
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| Kruislaan 409 | PO Box 41882 | ||
| 1098 SJ Amsterdam | 1009 DB Amsterdam | ||
| The Netherlands | The Netherlands | ||
| phone: +31 20 592 2000 | fax: +31 20 592 5155 | ||
Promotiebericht, Nikhef, 25-02-2008
Nikhef PhD student Joana Ferreira Montenegro finished her research on the direct measurement of the W boson mass. She used data of the former DELPHI experiment at the Large Electron Positron Collider (LEP) at CERN. The results were in very good agreement with the world average mass of this elementary particle. She will defend her thesis the 20th of March at the Radboud Universiteit Nijmegen.
The W boson is the charged mediator of the weak interaction which is one of the four kinds of interactions present in nature. The W boson is, for particles standards, a heavy particle. In addition, it lives for a very short time before it decays into other particles. These are the reasons why one does not observe W bosons in daily life.
The subject of this thesis is the direct measurement of the mass of the W boson in semi-leptonic events. In this kind of events one of the two W bosons decays into a quark and an anti-quark and the other decays into a lepton and its corresponding neutrino. This measurement was performed using the data collected by the DELPHI experiment at LEP at CERN, Geneva, using data collected at centre-of-mass energies ranging from 183 GeV to 206 GeV.
The events which passed a first selection by an Iterative Discriminant Analysis were subsequently analysed using the Ideogram method. The method aims to include the maximum obtainable amount of information in the event in order to minimise the statistical uncertainty. After extracting a W mass value and its statistical uncertainty a thorough analysis of the possible sources of systematic uncertainties was performed. A total of 14 systematic uncertainties were evaluated. The error due to the uncertainty in the knowledge of the fragmentation process was estimated in a model independent way using the Mixed Lorentz Boosted Z method. This method was used in semi-leptonic events for the first time in this thesis.
The final result for the W mass measurement in the semi-leptonic channel using a data sample with an integrated luminosity of 64 pb-1 was found to be Mw=80.455 ± 0.071 (stat.) ± 0.029 (syst.) ± 0.009 (LEP) GeV. This result is in very good agreement with the current world average Mw=80.398 ± 0.025 GeV.
Contact: joanafm@nikhef.nl