A search for $t\bar{t}$ resonances in the lepton plus jets final state with ATLAS using 14 fb$^{−1}$ of pp collisions at $\sqrt{s}=8$ TeV

A search for new particles that decay into top quark pairs ( $t\bar{t}$) is performed with the ATLAS experiment at the LHC using an integrated luminosity of 14 fb$^{-1}$ of proton-proton collision data collected at center-of-mass energy $\sqrt{s}=8$ TeV. The lepton plus jets final state is used, where the top-pair decays as $t\bar{t} \rightarrow W^{+}bW^{-}\bar{b}$ with one $W$ boson decaying leptonically and the other hadronically. The $t\bar{t}$ system is reconstructed using both a conventional resolved jet analysis and a large-radius jet substructure analysis. The $t\bar{t}$ invariant mass spectrum is searched for local excesses deviating from the Standard Model prediction. No evidence for a $t\bar{t}$ resonance is found and 95% CL limits on the production rate are determined for massive states in two benchmark models. The upper limits on the cross section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 5.3 pb for a resonance mass of 0.5 TeV to 0.08 pb for a mass of 3 TeV. A narrow leptophobic topcolor $Z'$ boson with a mass below 1.8 TeV is excluded. Upper limits are set on the cross section times branching ratio for a broad color-octet resonance with $\Gamma/m =$15.3% decaying to $t\bar{t}$. These range from 9.6 pb for a mass of 0.5 TeV to 0.152 pb for a mass of 2.5 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.0 TeV.