Abstract
A
bstract
A measurement of the associated production of a top-quark pair with the Higgs boson (
t
t
¯
H
) in multilepton final states is presented. The analysis is based on a data sample of proton-proton collisions at
s
=
13
TeV recorded with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 140 fb
−
1
. Six final states defined by the number and flavour of reconstructed charged leptons are combined in a simultaneous likelihood fit to extract the
t
t
¯
H
signal and constrain the most relevant backgrounds. The measured
t
t
¯
H
cross-section normalised to Standard Model (SM) prediction is
σ
t
t
¯
H
/
σ
SM
=
0
.
63
−
0.19
+
0.20
. This result corresponds to an observed (expected) significance of 3.3
σ
(5.3
σ
). Additionally, two other fits are used to measure the
t
t
¯
H
cross-section differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, and to extract the associated production cross-section of a single top-quark with the Higgs boson (
tH
) together with the
t
t
¯
H
one. The
CP
structure of the top quark-Higgs boson Yukawa coupling is probed through an analysis of
t
t
¯
H
and
tH
events. The results are compatible with the SM hypothesis, and values of the mixing angle between
CP
-even and
CP
-odd top-Higgs Yukawa couplings of |
α
|
>
62
°
are excluded at 68% confidence level.