Search for Higgs boson pair production in association with top-quark pairs using 196 fb−1 of proton–proton collision data at $$ \sqrt{s}=13 $$ and 13.6 TeV with the ATLAS detector

Abstract

A bstract This paper presents the first search for non-resonant Higgs boson pair production in association with a top-quark pair ( $$ t\overline{t} HH $$ 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 $$ \sqrt{s}=13 $$ s = 13 TeV and 56 fb − 1 at 13.6 TeV. The search targets three distinct final states expected from $$ t\overline{t} HH $$ 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 $$ t\overline{t} HH $$ t t ¯ HH production cross-section, relative to its Standard Model prediction, is measured to be $$ {\mu}_{t\overline{t} HH}=-{3}_{-12}^{+11} $$ 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 $$ t\overline{t} HH $$ t t ¯ HH production cross-section, with an expected limit of 21. The Higgs effective field theory Wilson coefficient $$ {c}_{t\overline{t} HH} $$ c t t ¯ HH is also constrained at the same confidence level to the range of $$ -3.9<{c}_{t\overline{t} HH}<3.3 $$ − 3.9 < c t t ¯ HH < 3.3 , compared with the expected range of $$ -4.0<{c}_{t\overline{t} HH}<3.5 $$ − 4.0 < c t t ¯ HH < 3.5 .