Test of Spin and Parity of the Higgs Boson in the 𝐻 → 𝑊 𝑊 ∗ → 𝑒𝜈𝜇𝜈 Decay Channel with the ATLAS Detector at the LHC

The Higgs boson takes a key position within the Standard Model and its underlying mechanism is crucial in order to explain why elementary particles and especially the heavy vector bosons have masses. Since its prediction in the early 1960s it took nearly 50 years until the existence of the Higgs boson could be proven. It was on 4th July 2012 that two experiments—ATLAS (“A Toroidal LHC ApparatuS”) and CMS (“Compact Muon Solenoid”)—at the Large Hadron Collider at CERN, the European Organization for Nuclear research, announced a 5 𝜎 excess verifying the existence of the Higgs boson. Its mass amounts about 125 GeV∕c 2 making the observation compatible with earlier exclusion limits. In January 2012 by the time the work on this thesis started, the confirmation of the Higgs boson existence was anticipated and further steps based on this hypothesis were contemplated. For this reason an analysis of the Higgs boson quantum numbers for spin and 𝐶𝑃 eigenvalue was initiated. These quantities are crucial to verify the agreement between the later discovered resonance and the Standard Model expectation. For the analysis the decay channel 𝐻 → 𝑊 𝑊 ∗ → 𝑒𝜈𝜇𝜈 was chosen, which features a high cross section times branching ratio in the anticipated mass regime. In spite of the two neutrinos in the final state the signature of the final state is clearly selectable and can rely on the precise missing transverse momentum reconstruction of the ATLAS detector. The contributions to the above publications comprise studies and development work for a wide range of aspects of the published analysis such as the definition of sensitive and backgroundenriched regions in the measured variable space, studies of multivariate techniques to optimise the sensitivity, support of the reweighting studies for the 𝐶𝑃 -mixing analysis, implementation and impact estimation of systematic uncertainties and further fields. The major focus though was set on Monte Carlo event generator comparison studies and the statistical evaluation to obtain final results taking background processes and systematic uncertainties into account. A unique feature of the thesis is the evaluation of a sophisticated automated “smart” binning of two-dimensional qualifier outputs from the applied multivariate analysis techniques. First exclusion limits for non-Standard-Model spin and 𝐶𝑃 properties of the Higgs boson could be set already in 2013 and further improved in 2015. The analysis presented in this thesis can exclude all tested spin-2 models at a confidence level of 82.5 to 99.1 %. Two non-Standard-Model 𝐶𝑃 scenarios (CP-odd and CP-even with higher dimensional couplings to Standard Model particles) can be excluded respectively at 97.1 and 64.5 % confidence level. Combining the results from all bosonic decay channels at ATLAS each tested spin-2 and 𝐶𝑃 scenario can be excluded at more than 99.9 % confidence level. The newly introduced “smart” binning procedure is shown to be working reliably and takes a stand as a promising tool for future high energy physics analyses.