  We present the result of a search for a massive color-octet vector particle,
(e.g. a massive gluon) decaying to a pair of top quarks in proton-antiproton
collisions with a center-of-mass energy of 1.96 TeV. This search is based on
1.9 fb$^{-1}$ of data collected using the CDF detector during Run II of the
Tevatron at Fermilab. We study $t\bar{t}$ events in the lepton+jets channel
with at least one $b$-tagged jet. A massive gluon is characterized by its mass,
decay width, and the strength of its coupling to quarks. These parameters are
determined according to the observed invariant mass distribution of top quark
pairs. We set limits on the massive gluon coupling strength for masses between
400 and 800 GeV$/c^2$ and width-to-mass ratios between 0.05 and 0.50. The
coupling strength of the hypothetical massive gluon to quarks is consistent
with zero within the explored parameter space.
