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

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