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A search for new particles that decay into top quark pairs is reported. Thesearch is performed with the ATLAS experiment at the LHC using an integratedluminosity of 20.3 fb$^{-1}$ of proton-proton collision data collected at acentre-of-mass energy of $\sqrt{s}=8$ TeV. The lepton-plus-jets final state isused, where the top pair decays to $W^+bW^-\bar{b}$, with one $W$ bosondecaying leptonically and the other hadronically. The invariant mass spectrumof top quark pairs is examined for local excesses or deficits that areinconsistent with the Standard Model predictions. No evidence for a top quarkpair resonance is found, and 95% confidence-level limits on the production rateare determined for massive states in benchmark models. The upper limits on thecross-section times branching ratio of a narrow $Z'$ boson decaying to toppairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0TeV. A narrow leptophobic topcolour $Z'$ boson with mass below 1.8 TeV isexcluded. Upper limits are set on the cross-section times branching ratio for abroad colour-octet resonance with $\Gamma/m =$ 15% decaying to $t\bar{t}$.These range from 2.5 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. AKaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded formasses below 2.2 TeV.

A search for $t\bar{t}$ resonances using lepton-plus-jets events in proton-proton collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector