Search for ttbar resonances in the lepton+jets channel in ATLAS using 14fb^{-1} of pp collisions at 8 TeV

The search for ttbar resonances that could be produced at the CERN Large Hadron Collider allow the investigation of a wide range of physics beyond the Standard Model. In such a scheme, the top quark is often produced with a transverse momentum that is large as compared to its mass. The decay of such highly boosted top leads to a topology that differs in several respects from that encountered when the top quarks are produced approximately at rest. In particular, for high mass ttbar with a lepton + jets final state, the distance between the three jets originating from the hadronic top decay ( t -> Wb -> q qbar b) becomes small, leading to an important jet merging. We present the analysis applied to the 2012 proton-proton collisions at 8 TeV in the center of mass recorded by the ATLAS experiment, aiming to search for ttbar resonance where the complete 3-body decay of the hadronic top is reconstructed as a single fat jet and identified by investigating its substructure.