Studies into measuring the Higgs CP-state in $H$ $\to$ $\tau\tau$ decays at ATLAS

Since the discovery of the Higgs boson in 2012, experimental particle physics made large efforts to measure all properties of the new particle to characterise all its properties as precisely as possible. So far, all observations of the Higgs boson are compatible with the predictions by the Standard Model (SM). However, not all properties are measured with sufficient precision. This thesis studies the Higgs bosons behaviour under Charge-Parity (CP) transformation in its decay to two $\tau$ leptons using 36.1 fb$^{−1}$ of LHC data recorded by the ATLAS experiment in 2015 and 2016. A violation of the CP symmetry is one of the three Sakharov criteria to explain the dominance of matter over antimatter observed in our Universe today. However, the CP violation observed in the quark and lepton sectors of the SM is yet insufficient to explain this imbalance. Therefore, an observation of a new source of CP violation in the Higgs sector would be of particular importance to address this open question in physics. At the same time, it would be a clear sign of new physics beyond the SM, which predicts only one pure CP-even Higgs boson and a conservation of the CP symmetry in the Higgs boson production and decays. In this dissertation the CP state of the Higgs boson is measured in dihadronic $H$ → $\tau\tau$ decays from the angle φ∗$_{CP}$ between the two $\tau$ decay planes. Any deviation from the CP-even nature of the SM Higgs boson CP would appear as a phase shift in the angular distribution. The presented approach is model-independent and allows for a direct measurement of CP violation in the Higgs coupling to fermions, which makes it unique among other Higgs CP measurements performed at the LHC. Another important part of this thesis is the validation of the applied methods in $Z$ → $\tau\tau$ decays. The Higgs CP measurement benefits substantially from the $\tau$ particle flow reconstruction developed by the ATLAS collaboration for run 2. This dissertation also comprises studies on improving the particle flow method applied to jets. Finally, a maximum likelihood fit is performed to determine the CP-mixing angle φ$_{\tau}$ in various signal and control regions. In this measurement, no sign of new physics has been observed. The measured CP-mixing angle of φ$_{\tau}$ = (10$^{+40}_{-35}$)$^{◦}$ is compatible with the SM expectation of φ$\tau$ =0$^{◦}$ within its uncertainties. This result allows to exclude a CP-mixing angle larger than 50$^{◦}$ and smaller than −25$^{◦}$ at 68% CL. A pure CP-odd Higgs boson can be excluded at 89% CL. Furthermore, estimates on the expected sensitivities are given for the full run 2 dataset and larger amounts of data, as they are expected by the high-luminosity LHC.