Abstract: We present a search for a new heavy particle X produced in association with a top quark, p¯p?t(X?¯tq) or p¯p?¯t(¯¯¯X?t¯q), 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 ¯t+j or t+j resonance in t¯t+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 (p¯p?Xt or ¯¯¯X¯t) 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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