Measuring the $\mathcal{CP}$ nature of the Yukawa coupling between the Higgs boson and tau leptons

The first direct measurement of the $\mathcal{CP}$ nature of the Yukawa coupling between the Higgs boson and tau leptons is presented. The data used in this analysis were collected in proton-proton collisions recorded by the CMS experiment at the LHC and correspond to a total integrated luminosity of 137 fb$^{-1}$ and a centre-of-mass energy of $\sqrt{s} {=}13$ TeV. $\mathcal{CP}$-sensitive observables are reconstructed using the angular correlation between the decay planes of tau leptons in the $H\to\tau^+\tau^-$ decay. The observed (expected) value of the effective mixing angle between the $\mathcal{CP}$-even and $\mathcal{CP}$-odd couplings is found to be $-1\pm 19^\circ$ ($0\pm 21^\circ$) at 68% confidence level. The result is in agreement with the Standard Model predictions within uncertainties and disfavours a pure $\mathcal{CP}$-odd coupling at 3.0 $\sigma$ confidence level.