Search for Heavy Higgs Bosons $A/H$ Decaying to a Top Quark Pair in $pp$ Collisions at $\sqrt{s} =$ 8 TeV with the ATLAS Detector

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 $\sqrt{s} =$ 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$\beta$, for $m_{A/H}$ > 500 GeV.