| Resumo: | We present a search for a new heavy particle X produced in association with a top quark, pp̅ →t(X→t̅ q) or pp̅ →t̅ (X̅ →tq̅ ), where q stands for up quarks and down quarks. Such a particle may explain the recent anomalous measurements of top-quark forward-backward asymmetry. If the light-flavor quark (q) is reconstructed as a jet (j), this gives a t̅ +j or t+j resonance in tt̅ +jet events, a previously unexplored experimental signature. In a sample of events with exactly one lepton, missing transverse momentum and at least five jets, corresponding to an integrated luminosity of 8.7 fb-1 collected by the CDF II detector, we find the data to be consistent with the standard model. We set cross-section upper limits on the production (pp̅ →Xt or X̅ t̅ ) at 95% confidence level from 0.61 pb to 0.02 pb for X masses ranging from 200 GeV/c2 to 800 GeV/c2, respectively.
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