Search for pair production of heavy top-like quarks decaying to a high- W boson and a b quark in the lepton plus jets final state at √s=7 TeV with the ATLAS detector

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 √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-transversemomentum 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 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.