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

A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb$^{-1}$ of proton-proton collision data collected at the centre-of-mass energy $\sqrt{s}=8$TeV. The lepton plus jets final state is used, where the top-pair decays as $W^+bW^-\bar{b}$, with one $W$ boson decaying leptonically and the other hadronically. The top quark pair invariant mass spectrum is examined for local excesses that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence level limits on the production rate are determined for massive states in four benchmark models. The upper limits on the cross-section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour $Z'$ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio fo r a broad colour-octet resonance with $\Gamma/m =$~15% decaying to $t\bar{t}$. These range from 4.8 pb to 0.09 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall--Sundrum model is excluded for masses below 2.2 TeV.