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Measurement of vector Boson polarisation in WZ production at the LHC with the ATLAS detector

Abstract : Within the scope of this thesis, the cross section of the associate production of a W and a Z boson is presented and the polarization will measured for the first time in a diboson channel.Proton-proton collision data from the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of 13 TeV from 2015 and 2016 with an integrated luminosity of 36.1fb-1 have been analyzed.In the measurement, only fully leptonic decays to either electrons or muons are considered.With the larger dataset of proton-proton collisions available, the measurement is for the first time not any more dominated by statistical uncertainties.On the cross-section measurement, a precision of 4.4% was obtained, this is competitive to the precision obtained in the measurement at a collision center-of-mass energy of 8 TeV.The main systematic uncertainties issue from the background estimation, the muon and the electron identification efficiency.Within the scope of this thesis, detailed studies on the electron identification efficiency, the optimization of the lepton selection in the WZ measurement and on the estimation of the reducible background in WZ events were performed.The electron identification efficiency was calculated using a Tag and Probe method on Z decays to electrons.The candidate electron isolation is used to discriminate the signal against the background.The agreement to the modelling of the electron identification efficiency in simulation was studied and correction factors were derived which were provided to the ATLAS collaboration.The lepton selection has been optimized for the rejection of non-prompt leptons in the WZ measurement, the remaining reducible background from these non-prompt events is estimated using the data-driven Matrix Method.The extracted cross section in the fiducial phase space is 8% higher than the prediction available at next-to-next-to leading order but still compatible with the predictionsBoson polarization measurements have been performed on single bosons, however, a measurement has never been done so far in a diboson channel.Angular distributions of the boson decay products have been shown to be particularly sensitive to polarization effects.The polarization is extracted by means of a maximum likelihood fit of templates representing the angular distributions of longitudinal, left- and right-handed transverse polarization to data.A fraction of 25% of longitudinally polarized W and Z bosons with an uncertainty of 7 and 4%, respectively, are measured in the fiducial phase space.A precision on the longitudinal polarization fraction of 15% can be obtained whereas the measurement is dominated by statistical uncertainties.
Keywords : ATLAS Dibosons LHC
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Angela Maria Burger. Measurement of vector Boson polarisation in WZ production at the LHC with the ATLAS detector. High Energy Physics - Experiment [hep-ex]. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAY067⟩. ⟨tel-02132816⟩

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