Search for t -tH/ARIGHTWARDS ARROW t -t t -t production in proton-proton collisions at SQUARE ROOTs = 13 TeV with the ATLAS detector

Abstract

A search is presented for a heavy scalar (H) orpseudo-scalar (A) predicted by the two-Higgs-doublet models,where the H/A is produced in association with a topquarkpair (t -tH/A), and with the H/A decaying into a t -t pair.The full LHC Run 2 proton-proton collision data collectedby the ATLAS experiment is used, corresponding to an integratedluminosity of 139 fb-1. Events are selected requiringexactly one or two opposite-charge electrons or muons.Datadrivencorrections are applied to improve themodelling of thet -t+jets background in the regimewith high jet and b-jet multiplicities.These include a novel multi-dimensional kinematicreweighting based on a neural network trained using dataand simulations. An H/A-mass parameterised graph neuralnetwork is trained to optimise the signal-to-backgrounddiscrimination. In combination with the previous search performedby the ATLAS Collaboration in the multilepton finalstate, the observed upper limits on the t -tH/A RIGHTWARDS ARROW t -tt -t productioncross-section at 95% confidence level range between14 fb and 5.0 fb for an H/A with mass between 400 GeVand 1000 GeV, respectively. Assuming that both the H andA contribute to the t -tt -t cross-section, tan β values below 1.7or 0.7 are excluded for a mass of 400 GeV or 1000 GeV,respectively. The results are also used to constrain a modelpredicting the pair production of a colour-octet scalar, with the scalar decaying into a t -t pair.