Measurement of the CP properties of Higgs boson interactions with τ-leptons with the ATLAS detector

A study of the charge conjugation and parity (CP) properties of the interaction between the Higgs boson and τ-leptons is presented. The study is based on a measurement of CP-sensitive angular observables defined by the visible decay products of τ-lepton decays, where at least one hadronic decay is required. The analysis uses 139 fb⁻¹ of proton−proton collision data recorded at a centre-of-mass energy of √s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Contributions from CP-violating interactions between the Higgs boson and τ-leptons are described by a single mixing angle parameter ϕ_τ in the generalised Yukawa interaction. Without assuming the Standard Model hypothesis for the H→ττ signal strength, the mixing angle ϕ_τ is measured to be 9^∘±16^∘, with an expected value of 0^∘±28^∘ at the 68% confidence level. The pure CP-odd hypothesis is disfavoured at a level of 3.4 standard deviations. The results are compatible with the predictions for the Higgs boson in the Standard Model.