Probing the Higgs boson CP properties in vector-boson fusion production in the $\textit{H → τ}$

The CP properties of the Higgs boson are studied in the vector-boson fusion production mode. The analysis exploits the decay mode of the Higgs boson into two $\textit{τ}$-leptons using 140 fb of proton-proton collision data at TeV collected by the ATLAS experiment at the Large Hadron Collider. Results are obtained using the Optimal Observable method. CP-violating interactions between the Higgs boson and electroweak gauge bosons are considered in the effective field theory framework, with the interaction strength described in the HISZ basis by , and in the Warsaw basis by , , and . No deviations relative to the Standard Model are observed, and limits are obtained on the strength parameters. The parameter is constrained to the interval [−0$\textit{.}$012$\textit{,}$ 0$\textit{.}$044] at the 95% confidence level while is constrained to [−0$\textit{.}$24$\textit{,}$ 0$\textit{.}$83], when considering both linear and quadratic effects of physics beyond the Standard Model.