Prospects for heavy Higgs boson searches in the top-quark pair decay channel with the ATLAS experiment at the LHC and at future high-energy colliders.

The existence of additional scalar and pseudoscalar Higgs-like bosons is predicted by several new physics models, particularly the so-called two Higgs doublet models (2HDM). In this framework, given the recent indications by the LHC experiments, one of the most appealing channels to search for the neutral heavy Higgs bosons is the top-quark pair decay channel. This study investigates the possibility of seeing such a heavy Higgs scalar/pseudoscalar boson through this decay channel at the ATLAS experiment by thoroughly analysing the Run 2 and Run 3 data and at the High-Luminosity LHC and future high-energy colliders. Monte Carlo simulations with different boson masses have been employed, varying the couplings with leptons and quarks, and analysed considering the signal-background interference effects.