Abstract: This Letter reports a search for a narrow resonant state decaying into two W bosons and two b quarks where one W boson decays leptonically and the other decays into a quark-antiquark pair. The search is particularly sensitive to top-antitop resonant production. We use the full data sample of proton-antiproton collisions at a center-of-mass energy of 1.96 TeV collected by the CDF II detector at the Fermilab Tevatron, corresponding to an integrated luminosity of 9.45??fb?1. No evidence for resonant production is found, and upper limits on the production cross section times branching ratio for a narrow resonant state are extracted. Within a specific benchmark model, we exclude a Z? boson with mass, MZ?, below 915??GeV/c2 decaying into a top-antitop pair at the 95% credibility level assuming a Z? boson decay width of ?Z?=0.012MZ?. This is the most sensitive search for a narrow q¯q-initiated t¯t resonance in the mass region below 750??GeV/c2.

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