UC Irvine

This paper presents the first search for non-resonant Higgs boson pair production in association with a top-quark pair (tt¯HH$$ t\overline{t} HH $$) 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$$ \sqrt{s}=13 $$ TeV and 56 fb−1 at 13.6 TeV. The search targets three distinct final states expected from tt¯HH$$ t\overline{t} HH $$ 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 tt¯HH$$ t\overline{t} HH $$ production cross-section, relative to its Standard Model prediction, is measured to be μtt¯HH=−3−12+11$$ {\mu}_{t\overline{t} HH}=-{3}_{-12}^{+11} $$. This result corresponds to a 95% confidence-level upper limit of 20 times the Standard Model prediction for the tt¯HH$$ t\overline{t} HH $$ production cross-section, with an expected limit of 21. The Higgs effective field theory Wilson coefficient ctt¯HH$$ {c}_{t\overline{t} HH} $$ is also constrained at the same confidence level to the range of −3.9