  In this report a new search for a narrow-width heavy resonance decaying into
top quark pairs (X -> ttbar) in ppbar collisions at sqrt(s)=1.96 TeV has been
performed using data collected by the D0 detector at the Fermilab Tevatron
collider. The analysis considers ttbar candidate events in the lepton+jets
channel using a lifetime tag to identify b-jets and the ttbar invariant mass
distribution to search for evidence of resonant production. The analyzed
dataset corresponds to an integrated luminosity of approximately 370 pb^-1.
Since no evidence for a ttbar resonance X is found, upper limits on sigma(X) x
B(X -> ttbar) for different hypothesized resonance masses using a Bayesian
approach are set. Within a topcolor-assisted technicolor model, the existence
of a leptophobic Z' boson with M(Z') < 680 GeV and width Gamma(Z') = 0.012
M(Z') can be excluded at 95% C.L..
