  The results of a search for the pair production of a fourth-generation
up-type quark (t') in proton-proton collisions at sqrt(s) = 7 TeV are
presented, using data corresponding to an integrated luminosity of about 5.0
inverse femtobarns 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 t' anti-t' 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.
