This Letter reports a search for non standard model topquark resonances, $Z'$, decaying to $t\bar{ t} \rightarrow W^+b W^-\bar{ b}$ %\rightarrow qq'b qq' \bar{ b}$, where both $W$ decay to quarks. We examine the top-antitop quark invariant mass spectrum for the presence of narrow resonant states. The search uses a data sample of $\ppb$ collisions at a center of mass energy of 1.96 TeV collected by the CDF II detector at the Fermilab Tevatron, with an integrated luminosity of 2.8 fb$^{ -1}$. No evidence for top-antitop quark resonant production is found. We place upper limits on the production cross section times branching ratio for a specific topcolor assisted technicolor model with width of $\Gamma_{ Z'} = 0.012 M_{ Z'}$. Within this model, we exclude $Z'$ boson with masses below 805 GeV$/c^2$ at the 95% confidence level.
Categories: hep-ex