We present a search for a new particle T′ decaying to a top quark via T′→t+X, where X goes undetected. We use a data sample corresponding to 5.7 fb-1 of integrated luminosity of pp̅ collisions with √s=1.96 TeV, collected at Fermilab Tevatron by the CDF II detector. Our search for pair production of T′ is focused on the hadronic decay channel, pp̅ →T′T̅ ′→tt̅ +XX̅ →bqq̅ b̅ qq̅ +XX̅ . We interpret our results in terms of a model where T′ is an exotic fourth generation quark and X is a dark matter particle. The data are consistent with standard model expectations. We set a limit on the generic production of T′T̅ ′→tt̅ +XX̅ , excluding the fourth generation exotic quarks T′ at 95% confidence level up to mT′=400 GeV/c2 for mX≤70 GeV/c2.
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