Master thesis defense by Jochen Heinrich – University of Copenhagen

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Master thesis defense by Jochen Heinrich

Title: Reconstruction of boosted 𝑊± and 𝑍0 bosons from fat jets

Abstract: We present the reconstruction of heavily boosted W and Z0 bosons from l parge R-parameter jets (fat jets) in all-hadronic proton-proton collisions at s = 8 TeV at the LHC. The electroweak gauge bosons are boosted to a degree at which their hadronic decay products are collimated enough to be reconstructed as a single fat jet. A mass-drop ltering procedure which is validated in studies on Monte Carlo (MC) samples is then applied to the fat jets with pT > 420 GeV to suppress pileup and soft radiation. W and Z0 bosons are identi ed based on their ltered jet mass. The eciency of common substructure observables and event shape variables, in distinguishing between signal and QCD background is evaluated on MC and the optimized observable selection used for the training of two boosted decision trees (BDT), in order to reduce the dijet background not originating from the decay of an electroweak gauge boson. For the rst BDT, signal MC has been trained against background MC, while for the second one the background is taken from sidebands in data. A peak corresponding to the mass of the electroweak gauge bosons can be observed in the filtered fat jet mass spectrum. The production cross section for hadronically decaying electroweak gauge bosons is measured. The data sample corresponds to 20.3 fb-1 collected with the ATLAS detector in 2012.