Measurement of $H{\to}W^\pm W^{\mp\ast}{\to}\ell^-\bar{\nu}_{\ell}\ell^{\prime+}\nu_{\ell^\prime}$ Decays at $\sqrt{s}{=}13$ TeV with the ATLAS Experiment and Analysis of Effective Higgs Boson Couplings

This thesis presents and discusses measurements of the coupling of the Higgs boson to vector bosons, using data collected at $\sqrt{s}=13$ TeV by the ATLAS detector. A full analysis of the first $5.8$ fb${}^{-1}$ of LHC Run 2 data investigating the $H{\to}W^\pm W^{\mp\ast}{\to}\ell^-\bar{\nu}_{\ell}\ell^{\prime+}\nu_{\ell^\prime}$ decay channel is presented, targeting the gluon fusion, vector boson fusion and Higgs strahlung modes of Higgs boson production with dedicated event selections. The analyses presented here include the first measurement of Higgs boson production via gluon fusion in this decay channel using ATLAS data at $\sqrt{s}=13$ TeV. A combined analysis of the $H{\to}ZZ^{\ast}{\to}\ell^{-}\ell^{+}\ell^{\prime-}\ell^{\prime+}$ and $H{\to}\gamma\gamma$ decay modes employing $14.8$ fb${}^{-1}$ and $13.3$ fb${}^{-1}$ of $\sqrt{s}=13$ TeV data, respectively, is also presented. No significant deviation from the Standard Model predictions is observed. The state-of-the-art techniques for obtaining physics predictions for such measurements are discussed, including different methods of obtaining continuous likelihood functions from a finite set of samples. This includes Matrix Element Reweighting as well as different methods of interpolation. The technique of Effective Lagrangian Morphing, which was developed in the context of this thesis, is derived from first principles, discussed in detail and supplemented with enhancements and methods to reduce the dimensionality and computational cost and to improve the accuracy of its predictions. A first application of this technique for a measurement of the CP-even and the CP-odd coupling parameters $g_{HVV}$ and $g_{AVV}$ in the $H{\to}ZZ^{\ast}{\to}\ell^{-}\ell^{+}\ell^{\prime-}\ell^{\prime+}$ channel is presented, and opportunities and challenges for future applications of this technique to measurements in the $H{\to}W^\pm W^{\mp\ast}{\to}\ell^-\bar{\nu}_{\ell}\ell^{\prime+}\nu_{\ell^\prime}$ channel are discussed.