Abstract
A test of CP invariance in Higgs boson production via vector-boson fusion using the method of the Optimal Observable is presented. The analysis exploits the decay mode of the Higgs boson into a pair of 𝜏
τ
leptons and is based on 20.3 fb−1
f
b
−
1
of proton–proton collision data at 𝑠√
s
= 8 TeV
T
e
V
collected by the ATLAS experiment at the LHC. Contributions from CP-violating interactions between the Higgs boson and electroweak gauge bosons are described in an effective field theory framework, in which the strength of CP violation is governed by a single parameter 𝑑̃
d
~
. The mean values and distributions of CP-odd observables agree with the expectation in the Standard Model and show no sign of CP violation. The CP-mixing parameter 𝑑̃
d
~
is constrained to the interval (−0.11,0.05)
(
−
0.11
,
0.05
)
at 68% confidence level, consistent with the Standard Model expectation of 𝑑̃=0
d
~
=
0
.
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