A study of H →ττ decays at the ATLAS Experiment

This thesis presents a measurement of the CP properties of the Yukawa coupling between the Higgs Boson and τ leptons. The Standard model predicts that CP symmetry is conserved when the Higgs Boson decays into a τ¯τ⁺ pair. An observable is constructed from the τ lepton decay products that is sensitive to the mixing angle of Higgs CP states, ϕτ. Two decay channels are used during this analysis, τhad−τhad, where both τ leptons decay hadronically and the τlep−τhad channel where one τ decays hadronically the other decays leptonically. This study was performed using 139 fb−1 of proton-proton collision data collected at the ATLAS experiment between 2015 and 2018 at a centre of mass energy, √s = 13 TeV. The Standard Model expectation value is 0 ±28◦at the 68% confidence level (CL) and -70/+75◦at the 95.5% CL. The measured value of ϕτ is 9 ±16◦at the 68% confidence level and ±34◦at 95.5% confidence level. The measured result disfavours the pure CP-Odd Higgs hypothesis to 3.4 standard deviations (σ). This observation is therefore consistent with the Standard Model.