Probing the Higgs boson CP properties in vector-boson fusion production in the H→τ⁺τ⁻ channel with the ATLAS detector

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 $τ$-leptons using 140 fb⁻¹ of proton-proton collision data at √s=13 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 d̃, and in the Warsaw basis by cHW̃, cHB̃, and cHW̃B. No deviations relative to the Standard Model are observed, and limits are obtained on the strength parameters. The d̃ parameter is constrained to the interval [$-0.012, 0.044$] at the 95% confidence level while cHW̃ is constrained to [$-0.24, 0.83$], when considering both linear and quadratic effects of physics beyond the Standard Model.