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 ˜d, and in the Warsaw basis by cH ˜W , cH ˜B , and cH ˜W B . No deviations relative to the Standard Model are observed, and limits are obtained on the strength parameters. The ˜d parameter is constrained to the interval [−0.012, 0.044] at the 95% confidence level while cH ˜W is constrained to [−0.24, 0.83], when considering both linear and quadratic effects
of physics beyond the Standard Model.