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We report the most restrictive direct limits on masses of fourth-generation down-type quarks b′, and quarklike composite fermions (B or T5/3), decaying promptly to tW. We search for a significant excess of events with two same-charge leptons (e, μ), several hadronic jets, and missing transverse energy. An analysis of data from pp̄ collisions with an integrated luminosity of 2.7fb-1 collected with the CDF II detector at Fermilab yields no evidence for such a signal, setting mass limits mb ′, mB>338GeV/c2 and mT5/3>365GeV/c2 at 95% confidence level. © 2010 The American Physical Society.

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Physical Review Letters