Search for Top-philic Heavy Resonances Using the Single-lepton Decay Channel in $pp$ Collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

A search for heavy resonances coupling exclusively to top quarks (top-philic) is presented. These resonances are predicted by extensions of the Standard Model of particle physics which, among other things, attempt to explain the Higgs boson mass in a ``natural'' way which does not require as much fine-tuning as is presently necessary. The analysis focuses on final states with three or four top quarks in the single lepton channel and makes use of $139~\textrm{fb}^{-1}$ of proton-proton collision data collected by the ATLAS detector between 2015 and 2018 at a center-of-mass energy of 13 TeV. No signs of new physics are observed and therefore limits are set on the production cross section of top-philic resonances with masses between 1 and 3 TeV. The observed (expected) limits range from 21 (14) fb to 119 (86) fb depending on the choice of model parameters.