  Many models containing particles which are candidates for dark matter, assume
the standard model particles and the dark matter candidates are mediated by a
spin-0 particle. At the LHC, one can use these models for dark matter searches.
One of the possible approaches for the search of these models is by considering
the decay of the spin-0 particle to a pair of $t\bar{t}$, thus modifying the
pattern of the top quark pair invariant mass spectrum. This search suggests a
good sensitivity in a parameter space different than the more traditional
searches. We examine this sensitivity and put limits on two benchmark models
containing candidates for dark matter, using previous ATLAS results. It was
found that when the mediator mass ($m_{Y_0}$) and the dark matter candidate
mass ($m_{\chi}$) have values of $m_{Y_0} \sim 2 \cdot m_{\chi}$, mediator
masses in the range of $[400,600]$ GeV are excluded. We compare our results to
direct detection experiments and show that we gain sensitivity for new regions
which are not covered by other searches.
