Cross Section Measurement of a Standard Model Higgs Boson in the $H \to WW \to l\nu l\nu$ Decay Channel with the ATLAS Experiment

The Standard Model of particle physics successfully describes the elementary particles and their interactions and has been tested to the utmost precision. The discovery of the Higgs boson at the Large Hadron Collider in 2012 marks the detection of the last remaining unobserved phenomenon. Thus, it confirms the mechanism of spontaneous electroweak symmetry breaking which represents the origin of the mass of elementary particles. The presented thesis examines the properties of a Standard Model Higgs boson in the decay channel H → WW → `ν`ν with the ATLAS detector at the Large Hadron Collider. The analyzed data has been recorded at a center-of-mass energy of √ s = 8 TeV and includes the complete 2012 dataset with an integrated luminosity of 20.3 fb−1 . The investigations focus on the dominant gluon fusion production process. On the basis of the particular signature of the Higgs boson decay, a general analysis strategy is developed to separate the signal from the background processes. The channel is characterized by two isolated leptons and large missing transverse momentum. With a dedicated event selection the significance of the measurement is successively increased. The accurate determination of the background processes is of essential importance and comprises Monte Carlo simulation as well as data-driven techniques. A statistical fit is used to optimize the complex procedures. This results in an observed signal significance of Z obs 0 = 4.69 σ for a Standard Model Higgs boson with a mass of mH = 125 GeV. The corresponding signal strength parameter amounts to ˆµ = 1.16 +0.31 −0.28. Finally, this value leads to the measurement of the Higgs boson production cross section, which is found to be σ obs pp→H→WW = 5.49 +1.46 −1.37 pb for the investigated √ s = 8 TeV proton-proton collisions at the Large Hadron Collider. Furthermore, exclusion limits on the Standard Model Higgs boson production can be determined for the mass range of 135 – 200 GeV with a confidence level of 95%. All the concluded results are in agreement with the Standard Model predictions.