Measurement of the $CP$ properties of Higgs boson interactions with τ leptons at the LHC

The ATLAS and CMS experiments have performed measurements of the $CP$ structure of the interaction between the Higgs boson and τ leptons using proton-proton collisions at √s~=~13~ TeV, delivered by LHC during 2016-2018. The measurements are based on the angular correlation between the decay planes of τ leptons produced in Higgs boson decays. The value of the effective mixing angle measured by ATLAS and CMS experiments are found to be 9 ± 16∘ and -1 ± 19∘ at the $68%$ CL, respectively. The data from ATLAS and CMS experiments disfavour the pure $CP$-odd hypothesis at $3.4$ and $3.0$ standard deviations, respectively. The results are compatible with expectations for the standard model Higgs boson.