Search for heavy particles decaying to pairs of highly-boosted top quarks using lepton-plus-jets events in proton--proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for new heavy particles that decay into top-antitop quark pairs ($t\bar{t}$) is performed using data collected by the ATLAS detector at the Large Hadron Collider from proton-proton collisions at a centre-of-mass energy of $13$\,\TeV{}. The integrated luminosity of the data sample is $3.2\,$fb$^{-1}$. Events consistent with top-quark pair production are selected by requiring a single isolated charged lepton, missing transverse momentum, and a high-transverse-momentum jet with substructure compatible with that of a hadronic top-quark decay. Jets identified as likely to contain $b$-hadrons are used to further control the background from other Standard Model processes. The invariant mass spectrum of the candidate top-quark pairs is searched for excesses above the background expectation. No significant deviations from the Standard Model predictions are found. Exclusion limits are set on the production cross section times branching ratio for hypothetical $Z'$ bosons decaying into $t\bar{t}$.