Search for tt̄ resonances in the lepton plus jets final state with ATLAS using 4.7 fb-1 of pp collisions at √s=7 TeV. Physical Review D - Particles, Fields, Gravitation and Cosmology Volume 88, Issue 1, 23 July 2013, Article number 012004

A search for new particles that decay into top quark pairs (tt̄) 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 tt̄→WbWb decay, the lepton plus jets final state is used, where one W boson decays leptonically and the other hadronically. The tt̄ 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 tt̄ invariant mass spectrum is performed. No evidence for a tt̄ 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.