Abstracts

Precise measurements of the properties of the top quark allow for testing the StandardModel (SM) and can be used to constrain new physics models. The top quark is predictedin the SM to decay almost exclusively to a W boson and b-quark. Thus, studying theWtb vertex structure at high precision and in detail is motivated.This thesis presents a measurement of the W boson helicity fractions in top quarkdecays with top/anit-top events in the lepton + jets final state using proton-proton collisions ata centre-of-mass energy of 8 TeV recorded in 2012 with the ATLAS detector atthe LHC. The data sample corresponds to an integrated luminosity of 20.2 fb-1. Theangular distribution of two different analysers, the charged lepton and the down-typequark in the W boson rest frame are used to measure the helicity fractions. The mostprecise measurement is obtained from the leptonic analyser and events which contain atleast two b-quark tagged jets. The results of F0 = 0.709 0.012 (stat.+bkg. norm.) +0.015 -0.014, FL = 0.299 0.008 (stat.+bkg. norm.) +0.013-0.012 and FR = -0.008 0.006 (stat.+bkg. norm.) 0.012 (syst.), which stand for longitudinal, left- and right-handed W boson helicity fractions respectively,are obtained by performing a combined fit of electron+jets and muon+jets channelsto data. The measured helicity fractions are consistent with the Standard Modelprediction. As the polarisation state of the W boson in top quark decays is sensitive tothe Wtb vertex structure, limits on anomalous Wtb couplings are set.Advisors/Committee Members: Quadt, Arnulf (advisor), Quadt, Arnulf (referee), Lai, Stan (referee).