Search for $t\bar t$ resonances in the lepton plus jets final state with ATLAS using 4.7 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 7$ TeV

A search for new particles that decay into top quark pairs (tt¯) is performed with the ATLAS experiment at the LHC using an integrated luminosity of 4.7 fb-1 of proton–proton (pp) collision data collected at a center-of-mass energy s=7 TeV. In the tt¯→WbWb decay, the lepton plus jets final state is used, where one W boson decays leptonically and the other hadronically. The tt¯ system is reconstructed using both small-radius and large-radius jets, the latter being supplemented by a jet substructure analysis. A search for local excesses in the number of data events compared to the Standard Model expectation in the tt¯ invariant mass spectrum is performed. No evidence for a tt¯ resonance is found and 95% credibility-level limits on the production rate are determined for massive states predicted in two benchmark models. The upper limits on the cross section times branching ratio of a narrow Z′ resonance range from 5.1 pb for a boson mass of 0.5 TeV to 0.03 pb for a mass of 3 TeV. A narrow leptophobic topcolor Z′ resonance with a mass below 1.74 TeV is excluded. Limits are also derived for a broad color-octet resonance with Γ/m=15.3%. A Kaluza–Klein excitation of the gluon in a Randall–Sundrum model is excluded for masses below 2.07 TeV.