Search for ttbar resonances in ATLAS

There are many theories trying to remedy the weaknesses of the Standard Model (SM). Some of these theories include new phenomena that couple preferentially with the top quark, due to its large mass of about 173 GeV. Predicting, for example, the existence of new resonances decaying into top quark pairs. A search for new heavy particles decaying into ttbar pairs in the lepton plus jets channel using ~200 pb-1 pp collisions at a center-of-mass energy of √s=7 TeV collected at the beginning of 2011 with the ATLAS detector has been performed. The models of new physics used as benchmarks in this search are a topcolor Z' boson and a Kaluza-Klein gluon gKK in Randall-Sundrum model. The main results are presented.