Abstract
A
bstract
This paper presents the first search for non-resonant Higgs boson pair production in association with a top-quark pair (
t
t
¯
HH
) using proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider. The data sample corresponds to an integrated luminosity of 196 fb
−
1
, comprising 140 fb
−
1
at a centre-of-mass energy of
s
=
13
TeV and 56 fb
−
1
at 13.6 TeV. The search targets three distinct final states expected from
t
t
¯
HH
decays: (i) one lepton (electron or muon) with at least five
b
-quarks, (ii) at least two
b
-quarks accompanied by two leptons with the same electric charges or multiple leptons, and (iii) at least three
b
-quarks with two photons. The
t
t
¯
HH
production cross-section, relative to its Standard Model prediction, is measured to be
μ
t
t
¯
HH
=
−
3
−
12
+
11
. This result corresponds to a 95% confidence-level upper limit of 20 times the Standard Model prediction for the
t
t
¯
HH
production cross-section, with an expected limit of 21. The Higgs effective field theory Wilson coefficient
c
t
t
¯
HH
is also constrained at the same confidence level to the range of
−
3.9
<
c
t
t
¯
HH
<
3.3
, compared with the expected range of
−
4.0
<
c
t
t
¯
HH
<
3.5
.