A Search for New High-Mass Phenomena Producing Top Quarks with the ATLAS Experiment

A search for top quark pair resonances in the lepton plus jets final states has been performed with the ATLAS experiment at the LHC. The search uses a data sample corresponding to an integrated luminosity of 33 pb$^{-1}$, and was recorded at a proton-proton centre-of-mass energy of 7 TeV. No evidence of a resonance is found. Using the reconstructed $t\bar{t}$ mass spectrum, limits are set on the production cross-section times branching ratio to $t\bar{t}$ for narrow $Z'$ models. The observed 95\% C.L. limits range from approximately 55~pb to 2.2~pb for masses going from $m=$ 500 GeV to $m=$ 1000 GeV. The analysis is also used to set limits on enhanced top quark production at high $t+X$ mass, using the production of quantum black holes to model the signal. In that context, enhanced $t+X$ production with a mass threshold below 2.35 TeV is excluded.