A search for new heavy particles that decay into top-antitop quark pairs (tt ̄) 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 36.1fb−1. Events consistent with top-quark pair production are selected by requiring a

single isolated charged lepton, missing transverse momentum, and jet activity compatible with

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 the production of hypothetical

Z′ bosons, Kaluza-Kein gluons, and Kaluza-Klein gravitons with subsequent decay into tt ̄