Measurement of the $CP$ properties of Higgs boson interactions with $\tau$-leptons with the ATLAS detector

The ATLAS collaboration
CERN-EP-2022-244, 2022.

Abstract (data abstract)
A study of the charge conjugation and parity ($CP$) properties of the interaction between the Higgs boson and $\tau$-leptons is presented. The study is based on a measurement of $CP$-sensitive angular observables defined by the visible decay products of $\tau$-lepton decays, where at least one hadronic decay is required. The analysis uses 139 fb$^{-1}$ of proton–proton collision data recorded at a centre-of-mass energy of $\sqrt{s}= 13$ TeV with the ATLAS detector at the Large Hadron Collider. Contributions from $CP$-violating interactions between the Higgs boson and $\tau$-leptons are described by a single mixing angle parameter $\phi_{\tau}$ in the generalised Yukawa interaction. Without assuming the Standard Model hypothesis for the $H\rightarrow\tau\tau$ signal strength, the mixing angle $\phi_{\tau}$ is measured to be $9^{\circ} \pm 16^{\circ}$, with an expected value of $0^{\circ} \pm 28^{\circ}$ 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.

  • Table 1

    Data from Figure 7


    Observed 1-D likelihood scan of the $CP$-mixing angle $\phi_{\tau}$.

  • Table 2

    Data from Figure 7


    Expected 1-D likelihood scan of the $CP$-mixing angle $\phi_{\tau}$.

  • Table 3

    Data from Figure 8


    Observed 2-D likelihood scan of the signal strength $\mu_{\tau\tau}$ versus the $CP$-mixing angle $\phi_{\tau}$.

  • Table 4

    Data from Auxiliary Figure 10


    Expected 2-D likelihood scan of the signal strength $\mu_{\tau\tau}$ versus the $CP$-mixing angle $\phi_{\tau}$.

  • Table 5

    Data from Table 6


    Free-floating parameters in the measurement.

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