Search for production in proton–proton collisions at with the ATLAS detector

A search is presented for a heavy scalar ($\textit{H}$) or pseudo-scalar ($\textit{A}$) predicted by the two-Higgs-doublet models, where the $\textit{H}$/$\textit{A}$ is produced in association with a top-quark pair and with the $\textit{H}$/$\textit{A}$ decaying into a pair. The full LHC Run 2 proton–proton collision data collected by the ATLAS experiment is used, corresponding to an integrated luminosity of Events are selected requiring exactly one or two opposite-charge electrons or muons. Data-driven corrections are applied to improve the modelling of the +jets background in the regime with high jet and $\textit{b}$-jet multiplicities. These include a novel multi-dimensional kinematic reweighting based on a neural network trained using data and simulations. An $\textit{H}$/$\textit{A}$-mass parameterised graph neural network is trained to optimise the signal-to-background discrimination. In combination with the previous search performed by the ATLAS Collaboration in the multilepton final state, the observed upper limits on the production cross-section at 95% confidence level range between 14 fb and 5.0 fb for an $\textit{H}$/$\textit{A}$ with mass between 400 and 1000 , respectively. Assuming that both the $\textit{H}$ and $\textit{A}$ contribute to the cross-section, values below 1.7 or 0.7 are excluded for a mass of 400 or 1000 , respectively. The results are also used to constrain a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a pair.