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

TL;DR

This paper measures the $CP$ properties of the Higgs boson interactions with $ au$-leptons using ATLAS data, finding results consistent with the Standard Model and disfavouring a pure $CP$-odd Higgs state.

Contribution

First measurement of $CP$-sensitive angular observables in Higgs-$ au$ interactions with ATLAS data, constraining the $CP$-violating mixing angle without assuming Standard Model signal strength.

Findings

The mixing angle $\, au$ is measured to be $9^ ext{ extdegree} \, extpm 16^ extdegree$.

Pure $CP$-odd hypothesis is disfavoured at 3.4 standard deviations.

Results are compatible with Standard Model predictions.

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.