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 CP ) properties of the interaction between the Higgs boson and τ τ -leptons is presented. The study is based on a measurement of CP CP -sensitive angular observables defined by the visible decay products of τ τ -leptons produced in Higgs boson decays. The analysis uses 139 fb ⁻¹ - 1 of proton–proton collision data recorded at a centre-of-mass energy of √s= 13 s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Contributions from CP CP -violating interactions between the Higgs boson and τ τ -leptons are described by a single mixing angle parameter φ τ ϕ τ in the generalised Yukawa interaction. Without constraining the H→ τ τ H → τ τ signal strength to its expected value under the Standard Model hypothesis, the mixing angle φ τ ϕ τ is measured to be 9∘ ± 16∘ 9 ∘ ± 16 ∘ , with an expected value of 0∘ ± 28∘ 0 ∘ ± 28 ∘ at the 68% confidence level. The pure CP 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.