Higgs boson phenomenology at the LHC

The outstanding performance of the Large Hadron Collider (LHC) led to the discovery of the Higgs boson in 2012. The paramount endeavour after this discovery is the examination of the Higgs-boson properties, amongst others the determination of its CP quantum number and total decay width. The experimental analysis of both properties requires precise theoreticalinput within the Standard Model of particle physics. Theoretical methods and predictions at next-to-leading-order (NLO) in perturbative Quantum Chromodynamics (QCD), addressing the CP nature and decaywidth of the Higgs boson, are presented in this thesis. The thesis is split intwo parts:The first part addresses the Caola-Melnikov method, which is utilised to constrain the Higgs width that is experimentally not directly measurable. Themethod relies on cross section measurements on and far off the Higgs boson peak.Two-loop corrections via a heavy top quark to the gluon-gluon initialised Zboson pair-production are examined as an expansion aboutthe heavy-top limit combined with a conformal mapping and Pade approximants. The impact of the full NLO QCD corrections to the signal and background cross sections, relevant for bounding the Higgs width, is investigated.The second part of this thesis examines how precisely the CP nature of the Higgs boson can be unravelled in its decay to tau lepton pairs. All subsequent major charged-prong decays of the tau leptons are included.The impact parameter method is utilised and allows to extract the CP-mixing angle of the Higgs boson from the distribution of a signed angle. NLO QCD predictions for the signal process as well as the Drell-Yan background, including a Monte Carlo simulation of measurement uncertainties, are computed. Energy and angular correlations of the charged prongs are analysed and used to suppress the Drell-Yan background contribution. In a second step, the sensitivity to the CP-mixing angle is increased by combining the impact parameter method with the rho-decay plane method for the decay channels where at least one tau lepton decays to a rho meson.Estimates for the achievable precision of the Higgs CP-mixing angle at the LHC are obtained