Probing the Standard Model Higgs boson in the WW decay mode with the ATLAS detector at the LHC

Sammanfattning: This thesis is concerns an analysis to probe the Higgs boson in the H→WW(')→lvlv decay mode, one of three analyses contributing to the discovery of the Higgs boson at a mass of mH = 125 GeV in July 2012. The dataset used are the proton-proton collisions collected with the ATLAS detector at the CERN Large Hadron Collider during 2012 at a centre of mass energy of 8 TeV, and correspond to a total integrated luminosity of 13.0 fb-1. An excess of events is observed in the data over the expected number of background events. The significance of the excess is estimated to be 2.8 standard deviations and the signal strength, defined as a ratio of the observed cross-section for the signal to the cross section predicted for a Standard Model Higgs boson with a mass of 125 GeV, is measured to be υ = 1:5 ± 0:6. At present, all results in the H→WW(')→lvlv decay mode concerning the newly discovered particle are compatible with the expectations from the Standard Model for a Higgs boson with a mass of mH = 125 GeV.In addition, an optimisation of the thresholds used to check the stability of the electronics calibration constants in the ATLAS Liquid Argon Calorimeter is presented. The calibration constants enters the calculation of the energy of the individual cells in the calorimeter and have to be validated regularly. The study is performed on calibration data from 2010 and 2011. New thresholds, based on the statistical behavior of the calibration constants, are determined separately for the different calorimeter regions.