  We discuss QCD radiative corrections to the production of a heavy neutral
resonance $Z'$ at the LHC assuming that it decays into a $ t\bar t$ final
state. Compared to previous studies, our computation includes top quark decays
as well as interference between the $Z'$ signal process and the QCD $t \bar t$
background. The interference contribution appears for the first time at
next-to-leading order (NLO) QCD and requires new one-loop amplitudes that are
not present when signal and background are treated separately. We describe some
examples of how QCD radiative corrections may influence both the exclusion
limits and studies of properties of the new resonance, once it is discovered.
