Abstract: The results of a search for the pair production of a fourth-generation up-type quark (t) in proton?proton collisions at ?s = 7 TeV are presented, using data corresponding to an integrated luminosity of about 5.0 fb?1 collected by the Compact Muon Solenoid experiment at the LHC. The t quark is assumed to decay exclusively to a W boson and a b quark. Events with a single isolated electron or muon, missing transverse momentum, and at least four hadronic jets, of which at least one must be identified as a b jet, are selected. No significant excess of events over standard model expectations is observed. Upper limits for the tt¯ production cross section at 95% confidence level are set as a function of t mass, and t -quark production for masses below 570 GeV is excluded. The search is equally sensitive to nonchiral heavy quarks decaying to Wb. In this case, the results can be interpreted as upper limits on the production cross section times the branching fraction to Wb.

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