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Abstract:

Inclusive, differential and production-mode cross-section measurements of the Higgs boson are performed in the H → ZZ* → 4ℓ decay channel. The analysis uses proton–proton collision data produced at the Large Hadron Collider at a centre-of-mass energy of s=13.6$$ \sqrt{s}=13.6 $$ TeV and recorded with the ATLAS detector, corresponding to an integrated luminosity of 164 fb−1. The inclusive fiducial cross-section for the H → ZZ* → 4ℓ process is measured to be σfid=3.65−0.33+0.35$$ {\sigma}_{\mathrm{fid}}={3.65}_{-0.33}^{+0.35} $$ fb, in agreement with the Standard Model prediction of σfidSM=3.68±0.17$$ {\sigma}_{\mathrm{fid}}^{\mathrm{SM}}=3.68\pm 0.17 $$ fb. Differential cross-sections are measured as a function of key kinematic observables of the Higgs boson and the four-lepton final state. Cross-sections are measured for the main production-modes in several exclusive regions of the Higgs boson production phase space and combined to measure an overall Higgs boson signal strength, defined as the measured cross-section normalised to the SM prediction, of μ = 0.99 ± 0.13. The results are interpreted in terms of modifications of Higgs boson couplings using the κ framework, within the Standard Model Effective Field Theory, and in scenarios probing the Higgs boson self-coupling. All the results are consistent with Standard Model expectations.

Abstract:

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