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Abstract

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 φτ in the generalised Yukawa interaction. Without constraining the H → τ τ signal strength to its expected value under the Standard Model hypothesis, the mixing angle φτ is measured to be 9◦±16◦, with an expected value of 0◦ ± 28◦ at the 68% confidence level. The pure CPodd 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.