Investigation of the CP properties of VBF Higgs production in hadronic final states of $H\rightarrow \tau\tau$ decays with the ATLAS detector

A test of CP invariance of the VBF Higgs production process is presented in the $\tau\tau$ final state. For this, a profile likelihood fit using a matrix-element observable method is employed to test whether a CP-odd component is present in the coupling. The analysis is carried out using 36.1 fb$^{-1}$ of proton-proton collision data recorded at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. It constrains the value of the CP violation inducing parameter $\tilde d$ to $[-0.090, 0.035]$ at the 68\% confidence level. This result is compatible with the Standard Model expectation of $\tilde d = 0$ and no CP violation. Particular emphasis is put on the final state including two hadronically-decaying $\tau$ leptons. Additionally, a measurement of the efficiencies of $\tau$-lepton triggers is presented. These triggers are necessary to collect the data on which the measurement of $\tilde d$ is based. Lastly, an ongoing measurement of Higgs-boson production cross-sections in the $\tau\tau$ final state using 139 fb$^{-1}$ of data is discussed, including an improved method of estimating background contributions in the final state with two hadronically-decaying $\tau$ leptons.