  A search is presented for production of a heavy up-type quark (t') together
with its antiparticle, assuming a significant branching ratio for subsequent
decay into a W boson and a b quark. The search is based on 4.7 fb^-1 of pp
collisions at sqrt(s)=7 TeV recorded in 2011 with the ATLAS detector at the
CERN Large Hadron Collider. Data are analyzed in the lepton+jets final state,
characterized by a high-transverse-momentum isolated electron or muon, large
missing transverse momentum and at least three jets. The analysis strategy
relies on the substantial boost of the W bosons in the t't'bar signal when
mt'>=400 GeV. No significant excess of events above the Standard Model
expectation is observed and the result of the search is interpreted in the
context of fourth-generation and vector-like quark models. Under the assumption
of a branching ratio BR(t'->Wb)=1, a fourth-generation t' quark with mass lower
than 656 GeV is excluded at 95% confidence level. In addition, in light of the
recent discovery of a new boson of mass ~126 GeV at the LHC, upper limits are
derived in the two-dimensional plane of BR(t'->Wb) versus BR(t'->Ht), where H
is the Standard Model Higgs boson, for vector-like quarks of various masses.
