Abstract
This paper presents a search for a new heavy particle produced in association with a top or antitop quark. Two models in which the new heavy particle is a color singlet or a color triplet are considered, decaying, respectively, to t_q or tq, leading to a resonance within the t__+jets signature. The full 2011 ATLAS pp collision data set from the LHC (4.7fb -1) is used to search for t__ events produced in association with jets, in which one of the W bosons from the top quarks decays leptonically and the other decays hadronically. The data are consistent with the Standard Model expectation, and a new particle with mass below 430 Gev for both W _ boson and color triplet models is excluded at 95% confidence level, assuming unit right-handed coupling. © 2012 CERN.