Search for Production of Heavy Particles Decaying to Top Quarks and   Invisible Particles in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV
  We present a search for a new particle $T'$ decaying to top quark via $T^{\prime} \rightarrow t + X$, where $X$ is an invisible particle. In a data sample with 4.8 fb$^{-1}$ of integrated luminosity collected by the CDF II detector at Fermilab in $p\bar{p}$ collisions with $\sqrt{s}=1.96$ TeV, we search for pair production of $T'$ in the lepton+jets channel, $p\bar{p} \rightarrow t\bar{t} +X+X \rightarrow \ell\nu b q q' b + X +X$. We interpret our results primarily in terms of a model where $T'$ are exotic fourth generation quarks and $X$ are dark matter particles. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and 600 GeV$/c^2$, the dark matter particle mass being anywhere below $m_{T^{\prime}}$. The data are consistent with standard model expectations, and we set 95% confidence level limits on the generic production of $T'\bar{T'} \rightarrow t\bar{t}+X+X$. We apply these limits to the dark matter model and exclude the fourth generation exotic quarks $T'$ at 95% confidence level up to $m_{T'}=360$ GeV$/c^{2}$ for $m_X\le 100$ GeV$/c^2$. 
CDF Collaboration
hep-ex