Search for tt¯resonances in the lepton plus jets final state with ATLAS using 4.7  fb-1 of pp collisions at √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 fb1 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 Z0 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 Z0 resonance with a
mass below 1.74 TeV is excluded. Limits are also derived for a broad color-octet resonance with =m ¼
15:3%. A Kaluza–Klein excitation of the gluon in a Randall–Sundrum model is excluded for masses
below 2.07 TeV.