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