A Search for $t\bar{t}$ Resonances in the Lepton Plus Jets Channel in 200 pb$^{-1}$ of $pp$ Collisions at $\sqrt{s} = 7$ TeV

A search for top quark pair ($t\bar{t}$) 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 200 pb$^{-1}$, and was recorded in 2011 at a proton-proton centre-of-mass energy of 7 TeV. No evidence for 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 and wide resonances. For narrow $Z'$ models, the observed 95\% C.L. limits range from approximately 38~pb to 3.2~pb for masses going from $m_{Z'}=$ 500 GeV to $m_{Z'}=$ 1300 GeV. In Randall-Sundrum models, Kaluza-Klein gluons with masses below 650 GeV are excluded at 95\% C.L.