Search for top-philic heavy resonances in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for the associated production of a heavy resonance with a top-quark or a top-antitop-quark pair, and decaying into a $t\bar{t}$ pair is presented. The search uses the data recorded by the ATLAS detector in $pp$ collisions at $\sqrt{s}= 13$ TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 139 fb$^{-1}$. Events containing exactly one electron or muon are selected. The two hadronically decaying top quarks from the resonance decay are reconstructed using jets clustered with a large radius parameter of $R=1$. The invariant mass spectrum of the two top quark candidates is used to search for a resonance signal in the range of 1.0 TeV to 3.2 TeV. The presence of a signal is examined using an approach with minimal model dependence followed by a model-dependent interpretation. No significant excess is observed over the background expectation. Upper limits on the production cross section times branching ratio at 95% confidence level are provided for a heavy $Z^\prime$ boson based on a simplified model, for $Z^\prime$ mass between 1.0 TeV and 3.0 TeV. The observed (expected) limits range from 21 (14) fb to 119 (86) fb depending on the choice of model parameters.