Search for a new scalar or pseudoscalar heavy Higgs Boson using production of four top quarks at the Large Hadron Collider

The Standard Model (SM) of particle physics has been remarkably successful in explaining a wide range of experimental observations. However, there are several fundamental questions which remain unanswered, such as the incorporation of gravity into the SM, radiative corrections to mass of Higgs boson, the matter-antimatter asymmetry, and the existence of viable dark matter candidates. One potential solution to these challenges is extending the SM by introducing an additional Higgs doublet. This analysis investigates the presence of a neutral scalar ($H$) or pseudoscalar ($A$) heavy Higgs boson, as predicted by the Two-Higgs-Doublet Model (2HDM), produced in association with a pair of top quarks and subsequently decaying into a top-quark pair. The analysis is based on proton-proton ($pp$) collisions at a center-of-mass energy of 13 TeV, using data collected by the ATLAS detector at the Large Hadron Collider (LHC). Events with exactly one or two oppositely charged leptons (electrons or muons) in the final state are considered. To enhance the modeling of the dominant background from top-antitop production with additional jets, data-driven corrections are applied. A powerful Graph Neural Network classifier is employed to improve signal-background discrimination. This search is further combined with a previous ATLAS analysis in the multilepton channel to set upper limits on the production cross section of $H/A$ bosons with masses ranging from 400 GeV to 1000 GeV in the alignment limit. Additionally, constraints are applied on a model predicting the existence of a color-octet scalar, known as a sgluon, which decays into a pair of top quarks.