A search for heavy pseudoscalar (A) and scalar (H) Higgs bosons decaying into a top quark pair $(t\bar t)$ has been performed with $20.3 fb^{−1}$ of proton–proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of $\sqrt{s}$ = 8 TeV. Interference effects between the signal process and Standard Model $(t\bar t)$ production, which are expected to distort the signal shape from a single peak to a peak–dip structure, are taken into account. No significant deviation from the Standard Model prediction is observed in the $(t\bar t)$ invariant mass spectrum in final states with an electron or muon, large missing transverse momentum, and at least four jets.
The results are interpreted within the context of a type-II two-Higgs-doublet model. Exclusion limits on the signal strength are derived as a function of the mass $m_{A/H}$ and the ratio of the vacuum expectation values of the two Higgs fields, tan β, for $m_{A/H}$ > 500 GeV.