  We consider the effects in the production via gluon fusion in LHC collisions
of one or two spin-zero new resonant particles $\Phi$ that decay into a top
quark pair. We revisit previous analyses of the interferences between the
heavy-fermion loop-induced $gg \rightarrow \Phi \rightarrow t\bar{t}$ signal
and the continuum QCD background $gg \rightarrow t\bar{t}$. We show that in the
presence of standard model fermions only in the $gg \rightarrow \Phi$ loops,
the interference effect is destructive causing a dip in the $t \bar t$ mass
distribution. Including New Physics such as additional vector-like quarks leads
to a totally different picture as the line-shape can be distorted by peaks and
dips. For the time being, the absence of such effects in ATLAS and CMS data
constrain models of the production and decays of the $\Phi$ state(s).
