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 -leptons produced in Higgs boson decays. 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_τin the generalised Yukawa interaction. Without constraining the H→ ττ signal strength to its expected value under the Standard Model hypothesis, the mixing angle ϕ_τis 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.