We present a search for a new particle T ′ decaying to top quark via T ′ → 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¯p collisions with √s = 1.96 TeV, we search for pair production of T ′ in
the lepton+jets channel, p¯p → t¯t + X + X → ℓνbqq′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/c2, the dark matter particle mass being anywhere below mT′ . The data are consistent
with standard model expectations, and we set 95% confidence level limits on the generic production