Measurement of triple Gauge-bosons couplings in e+e- collisions at LEP
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[S.l. : s.n.]
Number of pages
IV, 183 p.
RU Radboud Universiteit Nijmegen, 13 januari 2004
Promotor : Pohl, M. Co-promotor : Jong, P. de
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Experimental High Energy Physics
SubjectExperimental High Energy Physics
Many experiments have provided evidence that the Standard Model of electroweak interactions describes more than adequately the observed processes. In this theory, the dynamics are generated by requiring local gauge invariance under an SU(2)xU(1) symmetry. This also renders couplings between two W bosons and a neutral boson, the photon or Z boson, and are called triple gauge-boson couplings (TGC's). In fact, diagrams containing these vertices are necessary to ensure a proper high energy behaviour for certain processes, like W-pair production in e +e - interactions. The second phase of the Large Electron Positron (LEP) collider was devoted to running at centre-of-mass energies above the threshold for W-pair production, making it possible to study these couplings in a direct manner to great detail. This thesis describes the measurement of the TGC's using hadronic and semi-leptonic W boson pairs produced in e +e - collisions at the LEP collider. The data was recorded with the L3 detector from 1998 until 2000 with centre-of-mass energies ranging from 189 to 209 GeV. The most general Lagrangian for the trilinear vertex contains seven terms for each neutral boson. The measurement is made quantitative by introducing an arbitrary factor in front of each term. Under certain assumptions, the number of independent couplings is reduced to three. Values for these couplings are derived comparing measured with expected five-dimensional distributions. The results are combined with complementary L3 results and transformed into a measurement of the magnetic dipole and electric quadrupole moment of the W boson. These values also indicate that this boson is a point-like particle down to a scale of 10 -19 m. These measurements show that the TGC's indeed exist and their strength is in agreement with the predictions of the Standard Model. Moreover, the values for the couplings not foreseen in this theory are compatible with zero
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